Examine the following quotes:
"In order for a war to be just,
three things are necessary.
First, the authority of the sovereign....
Secondly, a just cause....
Thirdly ... a rightful intention."--Thomas Aquinas
"The act of self-defense can have a double effect: the preservation of one's own life; and the killing of the aggressor. . . . The one is intended, the other is not."
After examining these two quotes, Aquinas's basic belief is that murder is wrong, but it's not murder if you are attempting to preserve life. As it says above, preserving one's life is intended, killing the agressor is not intended. Now turn to the quote about war. It seems that Aquinas is saying a similar thing. For war to be justifiable, the person who initiates the war's power must be assessed, the cause must be just, and the intention must, ultimately, be good. This applies not just to war, but to any confrontational situation (such as self defense). The war's initiation is done by one's self--the cause is to preserve one's life--and the intention is just. Ultimately, in self-defense, the "war" is just.