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During WWII the US military was heavily segregated. Most black soldiers served in support roles such as truck drivers and stevedores. There were some combat black combat units such as the Tuskegee Airmen and 761st Tank Batallion.
A notable exception occurred during and after the Battle of the Bulge in December 1944. Faced with a shortage of replacements for front-line service General Eisenhower allowed black soldiers from support units to transfer into the infantry and join previously all white units.
As for Canada there were no segregated units in the Canadian military in WWII, but that doesn't mean there was no prejudice. Early in the war black volunteers were often rejected, especially by the Navy and Air Force.