Both sides accumulated large numbers of POWs, and, as often occurs in war, they were frequently exchanged. The exchange broke down, however, when the Confederacy refused to turn over black soldiers (both free blacks and former slaves who escaped to fight for the Union). They even threatened to enslave or execute them. The South also had prematurely re-enlisted some 30,000 men who had been paroled—freed from prison on the condition they not take up arms until an actual exchange had been completed.