The Outer Space Treaty keeps weapons of mass destruction out of orbit. That's not the same as prohibiting warfare in space. More than one nation has successfully tested destructive antisatellite weapons in space and many more are presumed to possess antisatellite capabilities. Meanwhile, important strategic capabilities such as early warning, secure communications, intelligence gathering, and command and control increasingly run through space. This raises the troubling possibility that the use of antisatellite weapons amid a crisis between nuclear-armed nations might lead to a nuclear exchange—indeed, US war games have repeatedly demonstrated that antisatellite weapons can cause crises to escalate in unpredictable ways. Below, experts debate this question: To what extent do antisatellite weapons increase the risk of nuclear war—and what can be done to moderate the risk?