Question

Why did the U.S. and Soviet Union agree to stop detonating bombs in 1963?

Answer 1

Because they did >33

Answer 2

Nuclear Test-Ban Treaty, formally Treaty Banning Nuclear Weapons Tests in the Atmosphere, in Outer Space, and Under Water, a treaty signed in Moscow on August 5, 1963, by the United States, the Soviet Union, and the United Kingdom that banned all tests of nuclear weapons except those conducted underground.