He was President of the Praesidium of the Supreme Soviet (i.e. titular head of state) (1960–64). As First Secretary of the Communist Party, he replaced Khrushchev (1964). Through these two offices he came to exercise effective control over Soviet policy, though initially he shared power with Kosygin. Brezhnev’s period in power was marked by economic and social stagnation, the intensified persecution of dissidents, and attempted détente, followed by renewed Cold War, in foreign affairs. He was largely responsible for the decision to invade Czechoslovakia in 1968, maintaining the doctrine that one socialist state may interfere in the affairs of another if the continuance of socialism is at risk.