When the U.S. was about to enter the war against the Nazis—no slouches in racial discrimination themselves—the question arose whether Black and white Americans could successfully fight together in integrated units. General George Marshall, the Army Chief of Staff, thought not. He was convinced that integration would jeopardize troop morale and discipline. He defended segregation as a "practical military expediency," not as approval of racial injustice, nudge-nudge, wink-wink (source).

According to the History News Network:

With these views, the military merely gave voice to widespread prejudices of the time. A July 1943 survey conducted by the Office of War Information showed that 96 percent of white soldiers from the South and 85 percent from the North insisted on segregation in the military. The attitude among African American GIs could not have been more different: 90 percent of Northern blacks and 67 percent of their Southern counterparts wanted to serve in integrated units. (Source)

During WWII, almost half of African American men who registered for the draft served in the armed forces (except for the Marines, who refused to admit Blacks at all until 1942). In 1947, President Harry Truman had appointed a panel to study more effective ways to protect the civil rights of American citizens, including in the military. The panel recommended a wide range of actions, like anti-lynching and anti-poll tax laws (can you believe they needed anti-lynching laws?), and strengthening the Civil Rights Division in the Department of Justice. Truman pushed Congress to pass these laws.

True to form, though, the southern senators threatened to filibuster any bills aimed at enacting the president's recommendations. So Truman used the powers granted to him by the Constitution to issue executive orders promoting civil rights. On July 26, 1948, Truman issued Executive Order 9981, finally abolishing segregation in all branches of the service:

It is hereby declared to be the policy of the President that there shall be equality of treatment and opportunity for all persons in the armed services without regard to race, color, religion, or national origin. (Source)

He added some muscle to the order by setting up the Fahy committee to study the procedures of the service branches and make sure everyone was getting with the program.

There was blowback from the military about this order. Lots of white servicemen still didn't want to have to eat, bunk, shower, and use the bathroom alongside African Americans. And what about mixed-race social events? Once you let Blacks dance with whites, who knows what might happen? The fear of miscegenation was alive and well in 1948.

Officers protested, too. But during the Korean War, the large number of casualties forced Black and white units to merge just to survive, and by 1953 the military was largely integrated out of necessity (source). Everyone seemed to adjust.

Truman was under pressure to integrate the military in 1948 for a couple of reasons. His opponent in the 1948 presidential race, Thomas Dewey, had endorsed integration in the military (source). Plus, how could a Jim Crow military be justified in occupying and bringing democracy to post-war Germany?

Hypocritical much?

Regardless, Executive Order 9981 was a bold strike against the "separate but equal" doctrine that had dominated social, educational, and military life, and it paved the way for the Brown decision and later civil rights legislation.

Fortunately, Truman never had to send in the troops to deal with the troops. That would have been awkward.