It's part of a broader shift in the Democratic Party as well as American society.

As you know, America goes to war in 1941 with a segregated army, which attracted not-inconsiderable criticism from A. Philip Randolph. Randolph and other civil rights activists had gotten a commitment from Roosevelt for desegregated federal war plants to help African American workers, and Randolph was similarly interested in doing so in the armed forces once the war ended.

Randolph is aided in this by the fact that African American units during the war performed equally as well as their white counterparts. Prior to this, there had been stereotypes about African American soldiers being unwilling to fight or being cowardly, but there's no evidence that supports on this. On top of this, internal army surveys reveal that white officers paired with non-white units overwhelmingly felt that the men under their command fought bravely, so at least in the lower echelons in the army, opposition to desegregation was eroding. The evidence on this point is ambiguous and not always supported by polling (then again, as we shall see, polling got it spectacularly wrong in 1948). And then, of course, there's the irony of the world's largest democracy going to war with a racially segregated army, which especially in light of the horrors of Nazi Germany's racial policies seems even more untenable. It doesn't help that there's a rhetorical war heating up with the Soviet Union, which never ceased making hay out of American racism and Jim Crow.

Ok, so that's going on in the background. We have to pay attention to the politics. Because of the New Deal, African Americans have begun shifting their political loyalty to the Democratic Party, but the Democratic Party is also the party of the Solid South and had historically stood behind Jim Crow. Northern cities such as Chicago but also including Cleveland, Minneapolis, Indianapolis, and others had all become home to growing African American populations who became increasingly important to the viability of the Democratic Party in major cities. Unsurprisingly, they wanted a stronger platform on civil rights around the country but especially in the South. Their hand is probably also strengthened by the war itself: having participated in a war for democracy, they would themselves like to be able to partake fully in it, and as the United States faced an uncertain future with the Soviet Union, alienating 13% of the country probably seemed like an increasing strategic mistake.

This is going on at the same time as a racial backlash in the South, where hardline white racists are all too aware that African American veterans were speaking up. This leads to a string of attacks on veterans, one of whom was violently blinded in a beating by a sheriff's deputy and which received national attention. This is pitting the two wings of the Democratic Party against itself: the Northern city democrats who are becoming more active on civil rights, and Boll Weevils (southern Democrats) determined to resist it.

All of this circles back to Truman. Truman himself was not free of racism. After all, he was born in Missouri, briefly joined the Klan in the '20s, and told jokes that would be considered racist today. However, Truman was a man whose view of morality was strikingly black-and-white (no pun intended): he decided if something was right or wrong, and he was horrified by the attacks on African American veterans. Truman idolized veterans above just above everybody else, and the poor treatment of African Americans apparently did rankle him considerably.

To that end, he had chaired a presidential committee on civil rights in 1946 that produced a publication, To Secure These Rights that offers a host of recommendations: federal anti-lynching laws, an anti-poll tax measure, a civil rights division in the Department of Justice, and of course, desegregation of the armed forces. He sends a recommendation that these be passed to Congress in 1948, but of course, he can't get very far: Southern Democrats will fight it tooth and nail.

He was also facing an election in 1948, and after 16 years of Democratic presidents, everybody assumed that a Republican would finally get in office. It doesn't help that Truman had been unable to pass much legislation since 1946, when the Republicans had made considerable gains in Congress, and so his image was of an inheritor president who had done relatively little.

So, we now head into the 1948 Democratic Convention. Civil rights is the major divisive issue going into this, and you get people like Hubert Humphrey who really wants a strong civil rights platform, and who endorsed Truman's actions. They're opposed by the Southern Democrats, but they're outvoted, and in response many of them stage a walkout and try to run a third-party, the so-called Dixiecrats. More on that later.

The convention runs from July 12 to July 14. On July 26, Truman issues Executive Order 9981 ordering the desegregation of the armed forces. He's able to do this because he can claim that the armed forces falls under the purview of the Executive Branch. With this, Truman has indisputably come out as hard as he possibly can for civil rights.

At the time, it looked like he was committing political suicide, driving away the most reliable sector of the Democratic party (historically, anyway). Observers at the time get it completely wrong: Truman realigned the Democratic party in such a way that African Americans became a pillar of the party, and in doing so, he wins key majorities in California, Illinois, and Ohio, which effectively hands him the election. He almost got a victory in New York, which was his opponent's home state. To be sure, the Dixiecrats do take away Louisiana, Mississippi, Alabama, and South Carolina, but these states were easily replaced by the margins from the three states I mentioned above.

So, it was part Cold War calculation, part moral consideration, and part political gamble. I think on the one hand Truman's decision can't be separated from politics: he wanted to win the '48 election, and African American votes were a critical margin. That being said, very nearly nobody, pollsters and journalists alike, thought he had a shot at victory, so if Truman did so solely for gain, he was either incredibly foolish and lucky, or one of the smartest politicians who ever existed.

I have a follow up: was there any contemporary opposition to the desegregation of the armed forces at the time?

Another follow up, how difficult a task was it to desegregate, was it an immediate or gradual thing and did it have a knock on effect on Americas underprepared state in the Korean war?