The Crazy World of Julius Vrooder (1974)

If they think you're crazy, you can get away with anything.

After returning home from the war, Vietnam veteran Julius Vrooder resides in a veteran's hospital, where he has been vaguely diagnosed as "psychiatrically impaired." Taking the news with an air of initially lighthearted defiance, Vrooder escapes and sets up camp under a highway, where he builds a paranoia-driven booby-trapped bunker for himself. Falling in love with not-so-bright nurse Zanni, Vrooder soon plans to elope with her to a remote Canadian outpost.