I listened to a talk between Ajahn Chah and a group of visitors headed by Sanya Dharmasakti (Chief Privy Councillor) from the late 1970s. Ajahn Chah was talking about the principle of kamma. Then one of the visitors, a military officer, asked about "doing one's duty" which might mean using violence sometimes. Ajahn Chah's reply was very direct: no matter if you call it "your duty" or not, if you use violence to kill living beings, it is definitely bad kamma. He emphasised that Dhamma and worldly laws are quite separate, that the law of kamma operates outside of the conventions of society. He kind of paused a little, because his visitors were high ranking Bangkok civil servants and officers, but then stressed again: you can't say that you haven't committed bad kamma by calling it "your duty". It may be necessary in order to keep law and order in society to use some harsh methods, but it is nevertheless within the sphere of kamma. He didn't make any flattering comments to them because of their social rank, he just gave them straight Dhamma using some down-to-earth similies.
On another occasion, some Westerner asked him, "How can all these terrible things be happening in Buddhist countries?" He replied: "This is not Buddhism -- it is people doing such things. The Buddha did not teach to do such things."
I found one good text on the subject of Buddhism and War:http://home.earthlink.net/~brelief1/bud_war.html
Recently I have been reading about the Buddhist history and how king Asoka in India for the first time used Buddhism as a kind of "national ideology" in order to govern the country and keep it under control and in peace. However, he first had to defeat some of his enemies and kill many people in the process. He also had to use punishment against those who were working against him. So it seems that there is a paradox in the very concept of a "dhammaraja" or Righteous King, and one cannot completely avoid using violence in that kind of position. Do you know the Temiya Jataka? It is the story of a prince who reflected on the law of kamma and decided to pretend that he was disabled, so that we would not have to take on the role of a king, because he was afraid of the bad kamma that he would get from sentencing people to death, etc. This Jataka story is very interesting because it is the opposite of the Cakkavatti-raja ideal (the Wheel-turning king) which is mentioned in some Suttas. I guess one tries to make a "heaven on earth" whereas the other acknowledges the real dangers of Samsara and shows the way out.