Why were African-Americans prevented from joining the armed forces, I've read that theres quite a lot instances of disenfranchised peoples of a society serfs,slaves,minorities being used as "cannon fodder" while the enfranchised are the cavalry/officers and what not, like colonial french armies and north africans, sepoys in india, slaves in greek city state armies etc
Why wasn't a similar policy practiced in America? Also why were the all japanese-american battalion allowed to serve when for all intents and purposes japanese americans were considered coloured people at the time too?
You are under a mistaken impression, it seems, that African-Americans did not serve in the US armed forces prior to the Vietnam War. They did. They served in significant numbers in every US war prior to that.
To list a few: the Civil War (around 200,000 Union soldiers were African-Americans, about 10% of the total), the Spanish-American War (2,500, again about 10%), World War I (about 350,000 were African-American, though many were not put in front-line positions; there were several "colored" divisions, however, like the famed "Harlem Hellfighters," that saw extensive combat, about 40,000 people total), World War II (125,000 African-American fighters, including the famed "Tuskegee Airmen," served in segregated divisions), and Korea (100,000, about 8% of total manpower, were African-American).
They were not "cannon-fodder" but they often were kept from positions of real influence. At various times the amount of explicit racism they experienced was high, and the frustrations of being honored for valorous service but returning home to humiliating conditions were plainly obvious. The US armed forces were desegregated in 1948, but it was only in the Vietnam era and later that serious efforts were made to have more African-American officers, etc.