On the fascinating site Letters of Note:
In December of 1944, whilst behind enemy lines during the Rhineland Campaign, Private Kurt Vonnegut was captured by Wehrmacht troops and subsequently became a prisoner of war. A month later, Vonnegut and his fellow POWs reached a Dresden work camp where they were imprisoned in an underground slaughterhouse known by German soldiers as Schlachthof Fünf (Slaughterhouse Five). The next month — February — the subterranean nature of the prison saved their lives during the highly controversial and devastating bombing of Dresden, the aftermath of which Vonnegut and the remaining survivors helped to clear up.
Vonnegut released the book Slaughterhouse-Five in 1969.
Below is a letter he wrote to his family that May from a repatriation camp, in which he informs them of his capture and survival: