“Take the high ground and hold it!” has been standard combat doctrine for armies since ancient times. Now that people and their machines have entered outer space, it is no surprise that generals the world over regard Earth orbit as the key to modern warfare. But until recently, a norm had developed against the weaponization of space—even though there are no international treaties or laws explicitly prohibiting nonnuclear anti­satellite systems or weapons placed in orbit. Nations mostly shunned such weapons, fearing the possibility of destabilizing the global balance of power with a costly arms race in space.