Transpose the case to one where you come across a body by the side of a rural road in your own country. You stop your car, find that the person is dead, then take her valuables and drive off. You are in no position to know whether the person has made a will or who the items should be returned to. Is your conduct then alright?
I would think not. Though you don't know who this person's friends and relatives are, its quite likely that she had friends and relatives, and it's quite possible that some of them would appreciate something to remember her by. Likewise, though you don't know whether or not she has made a will, she may well have done so and, in any case, will have legal heirs under the law. (One reason she may not have made a will is that she was content for her property to go to those the laws says it should go to if she dies intestate.) It's not your job to find the heirs, friends, relatives. But this does not mean you can just walk off with her stuff.
This reaction to the domestic case explains why your grandfather's conduct at least appears morally dubious. Just like the woman by the side of the rural road, the dead German is likely to have had friends and relatives and legal heirs. It is quite possible that the valuable watch -- perhaps passed down within the family -- would have been meaningful to one of them (much like your grandfather's watch might come to be your treasured possession one day).
The parallel might break down in two ways. First, one might argue that the Germans were enemies and therefore not worthy of any fine consideration. When we're bombing their cities, how can it be wrong to deprive them of a treasured watch? Here one needs to think further about whether it was alright to bomb their cities. Some argue that the ordinary constraints against bombing cities were overridden in this case by the military importance of the bombing raids. Even if this were right, no military purpose is served by stealing a watch from a dead German. Others might argue that the Germans had forfeited any claim to consideration. But how can we say this about those who opposed Hitler or about those who were children at the time (including perhaps the dead German's little son who, growing up half-orphaned, would have treasured his parent's watch)? And so it seems that the parallel to the domestic case still stands.
Second, one might argue that the watch would not have made its way to the dead German's heirs anyway. Had your grandfather not stolen it, another GI would have done so or else it might have been buried somewhere with the corpse. I cannot judge the truth or probability of these subjunctives. But, if this is what your grandfather reasonably thought, then why not ascertain the dead German's name and make an effort, later, to return the loot? Many soldiers have done exactly that and thereby contributed to reconciliation after the war, and even formed lasting friendships with their erstwhile enemies.
I am not so sure that you can get out of your military service simply by saying that you wish to preserve your liberty and don't wish to harm other people. You may know the present situation better than I do, but I know of a number of young Israelis who ended up in jail for refusing to serve in the IDF.
I see your justice point: The IDF is protecting the physical security of Israeli citizens (or at least of a large majority of the Israeli population to which you belong), and so it seems unjust for you to enjoy this protection but then also to refuse to contribute to it.
You say that there is nothing you can do to avoid being protected by the IDF. If this were true, then this would weaken your reasons to serve. To illustrate, suppose you have a fan who, unprompted by you, greatly improves your reputation by posting admiring stories about you on Facebook, by very effectively singing your praises to important people in your social environment whose support will greatly help your career, etc. You learn about this person's efforts, and you realize that you benefit from these efforts. But you have no obligation to reciprocate, I would think, precisely because you had no choice in the matter, never asked your fan to act in your behalf or even signaled your approval. Matters are different when you do have a choice and then actively take advantage of benefits made available to you. Suppose, for example, that all the other occupants of your apartment building collaborate to cultivate a beautiful flower garden near your building. This garden cannot be seen from the outside (it is surrounded by hedges), but because you love flowers you often go there and sit on the bench. Here it seems that it would be wrong of you to take advantage of the beautiful garden while refusing to join the effort to maintain it (assuming that your neighbors really want you to contribute etc.).
The preceding paragraph suggest that your justice reason for serving in the IDF is the stronger the more of a choice you have about whether to remain in Israel. If you could easily leave and, say, live in the US instead, then your remaining in Israel is closer to the garden case, where you are taking advantage of the efforts of others. If you have no realistic way of avoiding protection by the IDF (short of suicide, say, which is obviously not a serious option here), then your presence in Israel is closer to the Facebook case where you benefit without choice and thus may permissibly refuse to reciprocate by doing your fair share.
Coming to your first two reasons now. Serving in an army means subjecting oneself to the far-reaching authority of its commanders and political leaders. They may order you to kill people, and they may order you into situations in which you must kill in order to survive. Soldiers harm people; and, more importantly, soldiers often wrongly harm people. Many of the objectives armies are used to achieve are unjust objectives, and many of the people who get killed by soldiers are innocent people and people whose killing is not morally justifiable or excusable. So your fear that, by joining the army, you will become a participant in unjust harm is entirely realistic. Joining the IDF, you may well be ordered to man checkpoints and to police roads that stifle the movement of Palestinians within the West Bank, for example, or to fire tank shells at, or drop bombs upon, civilian homes in the Gaza Strip. To put it bluntly, you may be given orders that, if you comply, will make you harm and even kill innocent people whom it is wrong to harm or kill.
By joining the army, you will make it much harder for yourself to avoid wrongdoing, to avoid harming and even killing innocent people. How much this matters depends on the specific situation. If an army fights (or is disposed only to fight) justly for an important just cause, then one may have strong moral reason, on balance, to join it. If an army is fighting, or disposed to fight, for an unjust cause, then one may have strong moral reason to refuse to join even when one also expects personally to benefit from this army's success. In such a case one would still have reason to avoid the benefits if one can do so witout undue hardship and also reason, of course, to avoid making other contributions to the army's success.
So this is what you might say in defense of your refusal to join the IDF to others who are joining and accusing you of being a free-rider. You can say that one central objective the IDF is used to support is the appropriation of land in the West Bank for new and expanding Israeli settlements, and that this is an unjust objective and policy that wrongly harms the Palestinians who live there. It would be wrong to contribute to the injustice done to the Palestinians and therefore wrong for you to join the IDF. You might add that, while the IDF also serves the legitimate objective of protecting you and other Israeli civilians from violence by Palestinians, much of this protection is needed only because of unjust Israeli policy in the West Bank.
If it is said in response that you are benefiting from the settlement policy, you can respond that you are not actively taking advantage of it (e.g., by living in the West Bank) and only benefiting from it (e.g., through lower real estate prices in Israel) in ways you cannot reasonably avoid.
If it is said that you should then (given what you believe) actively avoid the benefits by working toward emigration, you might respond that you are willing to forego such benefits by donating them to organizations (such as B'Tselem, perhaps) that protect the human rights of Palestinians in the Occupied Territories. You might add that, as an Israeli citizen, you have a responsibility to work toward achieving greater justice of your country's social institutions and policies, and that the kind of discussions with your fellow citizens that are central to such work would be very much harder to conduct from abroad.
In conclusion, I think that -- if you see the situation in roughly the way I have guessed in the preceding three paragraphs -- you can avoid both: being a participant in injustice and unfairly benefiting from the efforts of others who have joined the IDF. My sense is, though, that your government will not make this path as easy for you as you seem to expect.
I think Leaman is right that, in war and elsewhere, one must often choose between morally unpalatable alternatives. For example, the only way to protect innocent people from being massacred may be an intervention that risks killing other innocent people. And then your question (how would you feel if one of your loved ones were killed as collateral?) would be balanced by a similar question on the other side (how would you feel if one of your loved ones were massacred because we decided not to intervene?).
Having said this, I also feel I understand what upset you in Leaman's answer. There are two relevant passages. One says that "try[ing] to distinguish as far as possible between civilians and insurgents .. is a pretty empty policy once the bullets start flying. Right now the US and British military in Afghanistan have responded to pressure from the Afghan government to unleash less remote bombing operations to cut down on civilian deaths, and this has resulted predictably in increased military deaths among the allies." I suppose an empty policy is one that is unlikely to be implemented. Still, in this case the empty policy is also the morally right policy, and we citizens should not let the Afghan government be the only one calling for restraint. We should support as firmly as we can the call that such restraints be implemented (which, as Leaman's last sentence suggests, has now been done to some extent). Distinguishing between civilians and insurgents will increase harm to our soldiers, and this is a real, terrible cost. But what right do we have to pursue a policy that preserves the lives and health of allied soldiers at the cost of vastly larger numbers of Afghan civilians (as was the practice under Bush)? In fact, fighting there, what right do we have to assign any greater a value to our soldiers than to local civilians?
The other passage suggests we give up "trying to establish a UN policy of regulating weapons that no-one would adhere to anyway." Here I am rather more optimistic than Leaman: I think the ban on the use of biological weapons has worked quite well and has prevented a lot of harm. And the effort to ban landmines (spear-headed by Princess Diane) has also been impressive in the broad support it has achieved, even though the key offenders have so far not budged. I do not think it is hopeless to achieve widespread agreement that cluster bombs and napalm must not be used in urban areas where many civilians live. In fact, most people seem to agree with this prohibition, and very few states continue to offend against it. To be sure, an urban operation becomes more costly for one's own soldiers if one cannot just drop napalm all over the neighborhood. But then the commanders will have to consider whether the operation is really worth the cost (something they may pay much less attention to when the costs of the operation will be borne by foreign civilians) and, if it is, risk the lives of the soldiers they command.
Looking at the history of warfare, it is clear that some armies have, even at considerable cost, conducted themselves well vis-a-vis civilians even at times when bullets were flying (or the fighting was intense) and other armies have bahaved horribly. In Vietnam and also in Iraq and Afghanistan, allied military policy has been informed by great indifference to the suffering of the civilian populations. Such indifference to civilians is especially appalling because of the crushing military superiority we enjoyed in these conflicts: the survival and territorial integrity of our (allied) contries were in not the slightest danger, and the free fire zones and the napalming of villages in Vietnam victimized deeply impoverished families who lacked both the will and the ability to do the slightest harm to the US or anyone. Instead of feeling compassion for such people and a commitment to keep them out of harm's way, our military responded to their poverty and defenselessness with racist and cultural chauvinism (which was also in evidence in the widespread recreational torture practiced by our troops in Iraq). This is not the way all armies behave, and it is not the way ours is destined to behave. And even if this were the way our army is, or all armies are, destined to behave, we should explain why this is wrong and why it would be wrong to join, or to allow oneself to be conscripted into, such an army
There are various potentially relevant differences. First, interventions abroad are often more likely to be counterproductive. The foreign government committing or condoning the human rights violations may be so powerful that the attempt to stop it will cause much more death and destruction than is now occurring. By contrast, we can bring overwhelming force to bear domestically and thereby crush even well-armed crime gangs.Second, internationally we do not have unique authority to judge and to act. We are just one of many similarly placed agents possibly able to do something. These agents (the governments of powerful states) are likely to see things differently -- e.g. may support different factions in a country that's facing a violent power struggle. For example, some potential interveners may believe that the Sri Lankan all-out assault upon the Tamil Tigers was a crime that had to be stopped (because so many civilians were also hurt and killed). Other potential interveners may believe that the assault was the only way to end the civil war once and for all. If one intervenes to enforce its view of the situation, another may go in to prevent that. This illustrates that a general permission to intervene is likely to make matters worse by leading to altercations among interveners. Third, governments are often disingenuous. If we endorse the kind of principle you are entertaining (let governments use military force when doing so can stop the attempted murders of civilians in other countries who peacefully advocate for human rights), then governments are bound to appeal to this principle -- even in bad faith -- to justify interventions that are really motivated by rather more self-interested motives. You may recall those Iraqi weapons of mass destruction that were mendaciously appealed to to justify the invasion of Iraq. It's just as easy mendaciously to appeal to attempted murders supposedly taking place in a country one would like to invade.You will note that these arguments are predicated on the world as it is. They do not deny that the world you suggest is desirable: a world in which people abused by their government can hope for protection from abroad. But building such a more desirable world requires changes that go deeper than the adoption of a principle of parity. In particular, it requires a single effective decision mechanism that judges in the manner of a court -- that is, in a rule-based and transparent manner -- whether some particular country is so badly governed that military intervention is justified. In addition, it requires a powerful alliance of states committed to giving effect to those judgments. These two conditions seem far off.
This question cannot be answered in general terms. Some killings that you may be required to perform may be justifiable, others not. Generally, killings in war are thought to be justifiable when two conditions are both fulfilled: Your country must have a just cause for being involved in the war in the first place. And each potentially lethal action within the war must be aimed at a legitimate target while taking great care to spare others who are not a threat. You must reassure yourself on both counts before you start killing people on the orders of others. This can be quite straightforward when you are ordered to defend your country against invading soldiers. But it can be far more difficult, if not impossible, when you are ordered to participate in an attack upon, and occupation of, another country.
Taking the U.S. invasion of Iraq as an example, there is considerable doubt about the first condition. The U.S. sought but failed to obtain UN Security Council authorization. The weapons of mass destruction supposedly held by Saddam Hussein's regime never turned up. There is no evidence for Hussein's alleged collaboration with Al Qaeda. Major human right violations committed by his regime were committed in the 1980's when the U.S. was actively supporting Iraq's war against Iran. Here you need to identify some cause and reassure yourself that it is sufficient to justify the attack on Iraq.
Regarding the second condition, various estimates put the number of Iraqi civilians killed as a result of the U.S. invasion at between 65,000 (Iraq Body Count) and 655,000 (the Lancet). Many were killed by local groups, to be sure, but only because the U.S. -- seeking to keep our troop commitment low -- failed to maintain public order and security. Many more were killed through indiscriminate U.S. bombardment of civilian areas from which hostile fire was supposedly received or to which suspected insurgents had supposedly retreated. Other civilians again were killed by U.S. troops taking revenge for losses suffered from roadside bombs or other attacks. (Last Saturday, 27 May 2007, for example, the Washington Post reported that: "Witnesses to the slaying of 24 Iraqi civilians by U.S. Marines in thewestern town of Haditha say the Americans shot men, women and childrenat close range in retaliation for the death of a Marine lance corporalin a roadside bombing.") If you conclude that the war is just, and decide to go to Iraq, you need to avoid being involved in the intentional or negligent killing of non-combatant civilians. If, on the basis of the evidence you have about the conduct of U.S. forces in Iraq, you anticipate that this would be very difficult or impossible to achieve, then you ought not to go in order to avoid being involved in unjustifiable killings.
We might wonder whether world peace would be so desirable. Isn't some conflict rather stimulating and exciting, and would it not be boring if everyone was in perfect harmony with everyone else? Of course, peace would be preferable to immense murder and destruction, however lacklustre it might turn out to be. On the other hand, an ideal state of affairs might be thought to include at least some conflict and aggression, and so the ideal of world peace may not be realized because few people actually want it to be realized.