The Space Force, the sixth and newest branch of the U.S. military, was authorized by Congress and signed into law by President Donald Trump in December 2019. The initiative had been shaped within the armed forces and Congress over the previous 25 years, based on the premise that as satellite and space technologies evolved, America’s military organizations had to change as well. From the start, the Space Force had detractors. Air Force officials wondered if it was necessary, while some political observers believed that it signified the start of a dangerous (and expensive) militarization of another realm. What seemed harder to argue against was how nearly every aspect of modern warfare and defense — intelligence, surveillance, communications, operations, missile detection — has come to rely on links to orbiting satellites. The recent battles in Eastern Europe, in which Russia has tried to disrupt Ukraine’s space-borne communication systems, are a case in point. And yet the strategic exploitation of space now extends well beyond military concerns. Satellite phone systems have become widespread. Positioning and timing satellites, such as GPS (now overseen by the Space Force), allow for digital mapping, navigation, banking and agricultural management. A world without orbital weather surveys seems unthinkable. Modern life is reliant on space technologies to an extent that an interruption would create profound economic and social distress. For the moment, the force has taken up a problem not often contemplated outside science fiction: How do you fight a war in space, or a war on Earth that expands into space? And even if you’re ready to fight, how do you make sure you don’t have a space war in the first place?