I always open up with how it fucked up my friends and their families. When I was a boy I worked for a woman pulling weeds and clearing her garden bed and digging irrigation ditches. Her father was a veteran of WWII, "the good war" as people like to think of it. She told me once that nobody could wake her father up by touching him. If you were within striking distance he would wake up throwing punches and trying to kill you. Even if you were his 10 year old daughter.

They would wake him up by grabbing a toe and leaving the room.

This was also a man who, if the stories are true, was among a couple dozen soldiers who blockaded some trucks at gunpoint so they could offload food supplies to the concentration camp they had just liberated. If all of that is true imagine the lives he saved. Imagine how much "good" he did with his time at war. And it still fucked him up until the day he died.

Once you get them onboard with the horror of war you can start to dismantle their ideas about the need for a military. It is disgusting that it takes PTSD riddled soldiers to get people to understand how bad war is and not the victims who died nameless under their guns. It really is awful that it takes soldiers coming home as broken humans for people to understand "war is bad." But, if that's the only angle you can work with you should use it. These are libs, afterall.