Jimmy Carter, Iran debacle. The current ME situation is a direct result of that.
John Kennedy, Bay of Pigs. The communist government of Cuba could have been easily toppled. We ended up with the Cuban missile crisis because of his vacillation.
Harry Truman. Mismanagement of Korea. Truman micromanaged the war effort and recalled MacArthur. North Korea has nukes and a starving populace. Allowing MacArthur to finish the job would have created a unified Korea.
Two of these Presidents, Carter and Truman, lost their chance at reelection because of their blunders. If the theory of Cuban involvement in Kennedy's assassination is true, it cost him more.
Another even more egregious blunder by Truman is his failure to prevent the Communist takeover of China in 1949. The Chinese people have been paying for that one ever since then, and we may wind up paying a huge cost as well.