In 1958, both the U.S. and Soviet Union agreed to informally suspend nuclear testing. However, this agreement was ended when the Soviets resumed testing in 1961, followed by a series of nuclear tests conducted by the U.S. These events led to much political fallout, as well as the Cuban Missile Crisis in 1962. It was felt that something had to be done to ease the significant tensions between these two countries, so on October 10, 1963, the Limited Test Ban Treaty (LTBT) was signed. This was an agreement between the U.S., the Soviet Union, and the U.K., which significantly restricted nuclear testing. All atmospheric, underwater, and outer space nuclear testing were agreed to be halted, but testing was still allowed underground. An additional 113 countries have signed this treaty since 1963.