After the death of Joseph Stalin in March 1953, relations between the Soviet Union and China began to deteriorate. Mao Zedong believed the new Soviet leader Nikita Khrushchev had made a serious error in his Secret Speech denouncing Stalin in February 1956, and criticized the Soviet Union's interpretation of Marxism–Leninism, in particular Khrushchev's support for peaceful co-existence and its interpretation. This led to increasingly hostile relations, and eventually the Sino-Soviet split. From here, Chinese communists played a decreasing role in helping their former allies because the Viet Minh did not support China against the Soviets.