What injury did Uncle Hammer get in the German war in Roll of Thunder, Hear My Cry?

Uncle Hammer got his leg blown off in the First World War. Like a lot of African Americans, Uncle Hammer believed that by fighting in the War, he could prove to the white men that black people were deserving of equality. Unfortunately, that's not how it turned out for him or any of the other black men he fought alongside.
Although Uncle Hammer has a good job and a pretty decent lifestyle up in Chicago, he's still acutely aware of the abiding bigotry and prejudice that still exist throughout the United States. In one particularly unpleasant example of blatant racism, Charlie Simms pushes Cassie over in the street simply because she accidentally bumped into his daughter. As a white man in a deeply segregated society, Simms thinks he's perfectly entitled to act this way. But Uncle Hammer's having none of it:

You think my brother died and I got my leg blown off in their German war to have some red-neck knock Cassie around anytime it suits him?