This postcard shows a stamp from the USPS 2005 release To Form a More Perfect Union: Seeking Equal Rights for African Americans. From the website:

The To Form A More Perfect Union stamps recognize the courage and achievement of the men and women who, during the years of the civil rights movement, struggled to bring the vision of our founding fathers closer to reality. Ten important milestones of the civil rights movement—Executive Order 9981, Brown v. Board of Education, Montgomery Bus Boycott, Little Rock Nine, Lunch Counter Sit-Ins, Freedom Riders, March on Washington, Civil Rights Act of 1964, Selma March and Voting Rights Act of 1965—are commemorated with the To Form A More Perfect Union stamps.

Executive Order 9981 integrated the U.S. military. The art is a silk-screen print by William H. Johnson called "Training for War."