A couple of comments.
First, some of the fathers of the Church, including Augustine, had your same concern. Out of this legitimate concern came what is called the Just War Doctrine. The bottom line is that killing in a war [if and when] that war is strictly for defensive purposes — after all other means of stopping an aggressor nation have been exhausted, there are reasonable prospects of success, and the unjust aggressor is causing grave and lasting damage — can in theory be justified.
In such a case, your primary intent is to save or preserve innocent human beings from an unjust aggressor. Whether or not any wars of the past have been truly just is another question. Before getting involved in armed conflict it is up to legitimate authority to calculate whether or not it is justified.
Secondly, you mentioned:
- Isn't preventing Catholics from joining an organization that openly kills millions of people a much more urgent matter than a woman having an abortion, where they can have no more than four children?
We have killed more human beings through abortion in this country then in all of our wars put together, times 100. The war on the unborn, unlike other wars, is very clearly unjust.