The U.S. Space Force will keep the U.S. lead in the growing militarization of space. However, the creation of the new military force received considerable criticism condemning it as a threat to the future of peaceful space exploration as required by international law. Indeed, the legality of the new military branch and its activities in space depends on whether it complies with the Outer Space Treaty of 1967. Specifically, Article IV of the Treaty, which delineates the allowed military activity in space. There are two main interpretations of ambiguous terms contained in the Treaty, such as “in orbit”, “install”, “station”, and “peaceful purposes”, which either restrain or broaden the activity of the U.S. Space Force in space. This article argues that the U.S. Space Force can legally expand its capabilities by appealing to the right to self-defense, which derives from one of the interpretations of peaceful purposes.