Basically for normal human beings to defend against enemies who are attacking, it's quite okay, but we defend without the intention to kill, we defend with the intention to stop someone creating harm. It's the intention behind what we are doing. When World War II was taking place, the US had no interest joining yet again another European war. That's how the US viewed it in 1936, 1939, 1940 and so on. The US had no interest in joining in it, it was tired of European wars. So it stayed out of the war. When it got attacked, it joined. Now fortunately for world history, the evil type attackers have always been defeated eventually.
Periodically throughout history there have been evil people who have tried to dominate the world. It's out of compassion for people to defend and try to stop the evil people. I mentioned the other day about police and armies, courts and judges, etc. That's all set up out of compassion for others. So if we can stop an aggressor, if we can stop someone trying to kill us, then that's a good intention. We want to hold on to that. Now at times, it means that people get killed. That's unfortunate, and for a person who takes on Buddhism to the extent that they become a monk or a nun, they will simply not get involved in a war at all. Because that's part of their vows, that's within the practice they are trying to maintain. For regular people, though, the Buddha advised them to "keep a strong army to defend themselves with." He actually advised kings to always maintain an army, to defend themselves against evil people. When you think of the precepts, it is not so much "not to kill", it's not to have the intention to kill. There's a slight difference there.