Though actively engaged in containment in Southeast Asia, the Middle East and Latin America, Johnson made it a priority to seek arms control deals with Moscow. The Soviet Union also sought closer relations to the United States during the mid-to-late 1960s, partly due to the increasingly worse Sino-Soviet split. Johnson attempted to reduce tensions with China by easing restrictions on trade, but the beginning of China's Cultural Revolution ended hopes of a greater rapprochement. Johnson was concerned with averting the possibility of nuclear war, and he sought to reduce tensions in Europe. The Johnson administration pursued arms control agreements with the Soviet Union, signing the Outer Space Treaty and the Treaty on the Non-Proliferation of Nuclear Weapons, and laid the foundation for the Strategic Arms Limitation Talks. Johnson held a largely amicable meeting with Soviet Premier Alexei Kosygin at the Glassboro Summit Conference in 1967, and in July 1968 the United States, Britain, and the Soviet Union signed the Non-Proliferation Treaty, in which each signatory agreed not to help other countries develop or acquire nuclear weapons. A planned nuclear disarmament summit between the United States and the Soviet Union was scuttled after Soviet forces violently suppressed the Prague Spring, an attempted democratization of Czechoslovakia.