four Warsaw Pact countries – the Soviet Union, Bulgaria, Poland and Hungary – invaded Czechoslovakia. The invasion comported with the Brezhnev Doctrine, a policy of compelling Eastern Bloc states to subordinate national interests to those of the Bloc as a whole and the exercise of a Soviet right to intervene if an Eastern Bloc country appeared to shift towards capitalism. The invasion was followed by a wave of emigration, including an estimated 70,000 Czechs initially fleeing, with the total eventually reaching 300,000. In April 1969, Dubček was replaced as first secretary by Gustáv Husák, and a period of "normalization" began. Husák reversed Dubček's reforms, purged the party of liberal members, dismissed opponents from public office, reinstated the power of the police authorities, sought to re-centralize the economy and re-instated the disallowance of political commentary in mainstream media and by persons not considered to have "full political trust". The international image of the Soviet Union suffered considerably, especially among Western student movements inspired by the "New Left" and non-Aligned Movement states. Mao Zedong's People's Republic of China, for example, condemned both the Soviets and the Americans as imperialists. Vietnam War U.S. President Lyndon B. Johnson landed 42,000 troops in the Dominican Republic in 1965 to prevent the emergence of "another Fidel Castro." More notable in 1965, however, was U.S. intervention in Southeast Asia. In 1965 Johnson stationed 22,000 troops in South Vietnam to prop up the faltering anticommunist regime. The South Vietnamese government had long been allied with the United States. The North Vietnamese under Ho Chi Minh were backed by the Soviet Union and China. North Vietnam, in turn, supported the National Liberation Front, which drew its ranks from the South Vietnamese working class and peasantry. Seeking to contain Communist expansion, Johnson increased the number of troops to 575,000 in 1968. North Vietnam received Soviet approval for its war effort in 1959; the Soviet Union sent 15,000 military advisors and annual arms shipments worth $450 million to North Vietnam during the war, while China sent 320,000 troops and annual arms shipments worth $180 million. While the early years of the war had significant U.S. casualties, the administration assured the public that the war was winnable and would in the near future result in a U.S. victory. The U.S. public's faith in "the light at the end of the tunnel" was shattered on January 30, 1968, when the NLF mounted the