I vaguely remember Ven. Dhammanando mentioning on here that the Buddha always forbid killing, but, for instance, Vinaya allowed a nun to beat off an attacker trying to rape her, in order to get away. This, however, does not apply to protecting any kind of "property," which monks wouldn't have anyway.
When it comes to self-defense, people often engage in violence for reasons of honor or protecting property. When it comes to the defense of one's own life, obviously it is justified, but with property, there is a grey area because such an action might simply be greed.
As for the war vs. pacifism argument, often it's a choice between two comparably regrettable outcomes and the decision lies, not with idealistic principle, but with what would practically reduce suffering for people.
WW2 was not simply "justified" or "unjustified". Both sides were pretty horrible, although I think that the outcome of an Allied victory is preferable to an Axis one, especially something like a global Nazi regime. This still doesn't mean, though, that it was "good" or "justified" for Allied soldiers to use horrible forms of torture, the carpet-bombing of Berlin, or the atomic bombs on Hiroshima and Nagasaki... In WW2, the Russians would actually train dogs to look for meat or other snacks under tanks. Then, out in the field, they would strap explosives to the dogs, the dogs would climb under enemy tanks, and blow up. Plus, WW2 created a wave of anti-German sentiment in America and set the stage for the Cold War with Russia.