Court-martialed?

They never show the court martial, he just says he was when he has the breakdown to Dahlia after being replaced. You might have missed the scene where he goes to the VA hospital and talks about being in captivity and that the Navy sent someone to visit his wife who counseled her about how bad her life might be when her POW husband comes home. He tells the therapist that they told his wife POW's lived only half as long as regular people, they had high rates of impotence and homosexuality and high rates of divorce. He tells the therapist that his wife left him and he doesn't get to see his children. Lander is angry because he must have broken down in captivity and signed a confession (which did happen in some cases during the actual Vietnam war) so when he came back he was possibly court-martialed for dereliction of duty or conduct unbecoming for signing the confession while in captivity. That's an assumption based on what he says in his rant to Dahlia. Add to the court martial he blames the Navy for turning his wife against him and the poor way he is treated by the VA hospital, that makes him very angry at America for the everything that happened to him during and after the war. That's why he wants to get even with America and all the regular people who "cheer court-martials with their two weenies and a coke" by blowing them up at the Super Bowl.