The Cuban invasion force, the Tibetan army, and the Viet Nam counter insurgency.
In short order he dropped the Cubans onto the beach into the waiting arms of Castro's army. He abandoned the Tibetans to the Chinese, and he pulled Lansdale out of Viet Nam and “accidentally” assassinated Diem.
Kennedy was a disaster.
He may of had some problems, but all was completely overshadowed when he stood firm against the Soviets, that put this planet as close to total nuclear war, than many people probably realize.
During the missile crisis, off the coast of Cuba, Kennedy ordered one of our ships to depth charge a Soviet Sub....It was not known until much later, that submarine we were dropping depth charges on, was armed with nuclear missiles....It's reported a near brawl on board the Soviet sub broke out, over whether to launch or not. If they had, a total nuclear exchange would have taken place.
That close.