You should read up on just war theory. Saint Augustine was a pretty big advocate of just war theory, and it aligns pretty well with the Church and Christ's teaching.

I half disagree. What constitutes as defense? Does saving other people count as defense?

From 1975 to 1979, the Khmer Rouge regime under Pol Pot brutally murdered almost 2 million people. This was roughly 25% of Cambodia's population at the time. You can google Cambodian genocide and see how horrible the treatment was. Some people would say it was worse than the Holocaust with how bad they were treated.

Vietnam, without any foreign support, launched a full-scale invasion of Cambodia. Vietnam's war with Cambodia lasted over 14 years and they took out the Khmer Rouge regime and stopped them from killing off more people.

Wouldn't you say Vietnam was justified in being the aggressor here? Granted, you could argue this is defense on behalf of someone else.