There are some things worth fighting for

Human history is nothing if not a narrative of constant struggle. Indeed, the miracle of culture, the domination of Earth's landscape, and the establishment of governance were all built on the weary backs of hard working men and women. Because we have worked so hard for our culture and our belongings and our government, we cherish them greatly. War is simply the manifestation of the idea that these are the things in life worth fighting for. The action of combating a threat to what you feel is right or what you know is yours is a valiant one, regardless of how block-headed it may sound at face value. It is only when we pervert the meaning of war for greed and hatred that it becomes unacceptable.

To compare WWII or the Revolutionary war to the Vietnam war or the Crusades is to make that distinction. When Hitler launched his morally unjustifiable agenda and began to take our allies, apathy was not optional; we had to fight, not only to protect ourselves, but also to preserve our ideals. When we felt entitled to our own governance and king George wouldn't budge, we absolutely had the right to take it by force. By contrast, the clash of ideals that lead to the death of thousands over conflicts that could have been easily resolved with diplomacy in the USSR and its proxy state of Vietnam were deplorable. Similarly, the senseless killing of middle-easterners and Catholics over misinterpreted words and invisible men in the sky c. 1200 AD could not have been less defensible. As long as it is merely protective of values, people, and possessions and not just a political con, war is morally defensible. However, there is a line, and it can be crossed without too much effort.