The United States, United Kingdom, and Soviet Union signed the Outer Space Treaty on this day in 1967. Its forebear, the 1959 Antarctic Treaty, effectively forbade the use of that continent for anything other than scientific research, including preventing any military activity there. The Outer Space Treaty was designed to do the same thing, but in (surprise!) outer space. This agreement, though, had a slightly different focus; it was expressly designed to prevent countries from putting any weapons of mass destruction in space. It also states that celestial bodies are to be used only for peaceful purposes.
At first, the U.S. and Soviets disagreed on how broad the treaty should be. The U.S. draft developed in 1966 covered only celestial bodies, while the Soviet proposal covered all of outer space. The latter draft won out, and after ironing out some minor details in private, the two Cold War powers (and the U.K.) signed the document. That helped usher in a collaborative approach to space exploration between countries on both sides of the divide that resulted in more ambitious projects like the International Space Station down the road.
You can read more about the Outer Space Treaty, and its text, in this article from the State Department.
