I've heard of a man who was recently diagnosed with ALS, who is volunteering to go fight. If you can make it to a neighboring country and cross the border, you can fight.

The Russian army is not noted for their congeniality. There are already reports of them raping and killing Ukrainian women and girls. Any American mercenaries caught could expect little sympathy or could be used as propaganda for Putin and his generals. But for soldiers, adrenalin is the rush of combat, and Ukraine needs the help. I wish them the best and know there are volunteers from other countries going into the fight.