John at The Lewis Crusade has an explanation of the Catholic moral position on this.
Catholics are sometimes criticized for treating all moral questions as black and white.
I think that that is a black and white judgement.
Because Catholics don't always do that.
It's just that on a number of liberal pet policies in general-- and abortion on demand specifically-- we do treat the moral issue as black and white.
It is never morally acceptable to intentionally provoke the death of another innocent human being.
That is a "black-and-white" position. No exceptions allowed.
That is why abortion-- as the Church defines it-- is never morally allowed.
The question of killing in war is different. Why?
First off, sometimes it is morally permissible to engage in a collective armed conflict to defend one's country or another country.
That should be obvious.
The second is because it is of a collective nature, and the intentions, knowledge and attitude of various actors come into play.
The decision to abort is based on the decision of a woman, or someone in her entourage, and perhaps the doctor.
Therefore, we're talking about a small number of actors involved in one act.
The process of engaging in war is complex. It involves a number of decisions, a lot of possible unknowns, etc.
Abortion is a "simple" act-- in the sense that it consists of one gesture.
War is a complex act-- in the sense that in involves countless gestures.
That is why Catholics are quick to condemn abortion, but not always quick to condemn war.
In plain speech-- it's not a black and white issue.
I'm sure that I will be criticized for treating abortion as black and white, but not war.
Really, the underlying reason for that criticism is not that I treat some issues as black and white.
It's that I don't agree with the liberal position.
Because liberals can be just as "black and white" too. While they pretend they aren't.
