Why does King believe that blacks in America have been the veterans of suffering?

The full line from Martin Luther King's speech is "you have been the veterans of creative suffering." To understand what he means, it is important to read the whole paragraph. He is pointing out to his fellow African Americans that they are not strangers to abuses and injustices. They have suffered at the hands of those in power in a number of ways and for a very long time. They are "veterans" because this is a shared experience that African Americans have endured for generations. The line also suggests that there were many different ways in which African Americans have suffered. They have been jailed, denied the right to vote, denied the use of public and private facilities, and robbed of their dignity. King is saying that these abuses have gone on for far too long. He is trying to tell his listeners that they must continue the struggle even in the face of repeated abuses and indignities.