The actor visited the State Department Thursday to discuss nuclear disarmament efforts and speak out against congressional efforts to slash billions of dollars in foreign assistance.
The 67-year-old told reporters, "People don't really take the time to understand just how important diplomacy is: It takes a long time. It's quiet. But it certainly is a lot cheaper than a war."
Douglas, who campaigns against atomic weapons for the U.N. and the anti-nuclear group Ploughshares, said that disarmament efforts have bogged down since a U.S.-Russian nuclear arms reduction treaty took force in February.
President Barack Obama also supports a global ban on nuclear testing, but that treaty lacks sufficient Senate support.
The Associated Press contributed to this report.