ON MARCH 11, 1778, Lt. Gothold Frederick Enslin became the first man to be booted out of the American military for homosexuality. As a sign of his disgrace, his sword was broken in half over his head. Then he marched out of George Washington's encampment at Valley Forge as drummers beat a slow, melancholy rhythm.

Soldiers have been drummed out of the U.S. armed services since the Revolutionary War. Most left the military like Lieutenant Enslin—quietly and in shame. Not until after World War II did some dishonorably discharged soldiers band together in New York in one of the country's first gay organizations, the Veterans Benevolent Association. At the time the group could offer little more to its members than moral support and friendship.

By the 1940s, the military classified homosexuality as a mental illness. In 1943, the military officially...