There's a special issue of "Marianne", a french magazine, concerning wars in history and how they started.
And I've translated an interesting part about philosophers and war.

And the causes of war, what are they ?
To explain war through its causes is deceiving. There are too many and every philospher defends its own thesis. Kant thought about the origins of war in nature, Hegel in history, Proudhon in poverty, Jaurès in capitalism. A matter of demonstration. And what then ? There are thousands of reasons to claim a territory. The war machinery is most of the time exterior to the State, rather than interior to its running.
War is a burst and burst always have an end. So ? It’s not our relationship to law that we have to rethink , but our relationship to the State, to power, to politics, to emancipation. The State doesn’t protect us from death. It’s the institutional form that takes us to a regulated death. It can be laudable to die for a country, but the country can also be misguided. There are thus better things to do than to uncouple power and peace, the State and the Law. It would be better to index politics on peace. To split from the new historicism of moral progress. Not to believe anymore in history’s rationality, but to trust politics a little more. Politics for all and not just the State. Tolstoï said « Power is the force that drives people. Tolstoï was the most suspicious thinker about strategies, chiefs, command in all its forms.
How can one be Tolstoïan ? We could as long as the enemy was another State, we can’t anymore if the enemy is everywhere and nowhere. We cannot literally wage war on terrorism. We can fight it, indeed, but we cannot make peace with terrorism. Peace and war are in this matter, not directly related to crime and law.
Either we make peace or we wage war. But you can’t do both together We should thus arrange power to a peace policy. We should pledge next to the State, not against it but next to it.
No philosopher can impose it, but people can, like in South Africa, so that the State can play its role and citizens their too.
It’s the minimal condition for the war empire to withdraw. Actually, there are no absolute positions on war or peace. Only limited use of one or the other. And a thought powerful enough to rule them. Lots of time and calm are needed to think about it.

Raoul