Simplify the following text for a Grade 10 native English-speaking person.
Original: In the late 1940s , lack of mutual trust was preventing the United States and the Soviet Union from making ground towards international arms control agreements , but by the 1960s steps were being taken to limit both the proliferation of nuclear weapons to other countries and the environmental effects of nuclear testing . The Partial Test Ban Treaty ( 1963 ) restricted all nuclear testing to underground nuclear testing , to prevent contamination from nuclear fallout , while the Nuclear Non-Proliferation Treaty ( 1968 ) attempted to place restrictions on the types of activities which signatories could participate in , with the goal of allowing the transference of non-military nuclear technology to member countries without fear of proliferation .
Simplified: When countries get or try to get nuclear weapons , it is called nuclear proliferation .