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His parents were communists, and, after the fall of the Hungarian Soviet Republic in 1919, the family fled to South America before immigrating to the Soviet Union. Steinmetz became a member of the Komsomol – the Soviet Communist Youth Organization, and then fought on the Republican side in the Spanish Civil War, becoming a captain in the Red Army during World War II.
In December 1944 (during the Battle of Budapest), when Soviet forces had encircled the Nazi German-controlled Hungarian capital, he delivered the ultimatum demanding the Germans and Hungarians to surrender. He was killed before the Soviet takeover of the city, when his car ran over a mine on the Üllői avenue in Pestszentlőrinc (today part of Budapest).