After spending 674 days in space, the military space plane known as the X-37B returned to Earth in October 2014. But no one really knows what its purpose was, or what it had been doing all that time, leading to all kinds of guessing in the popular press. Photos show something that looks like a baby space shuttle, and television newscasts suggested it could be a space bomber or a satellite meant to spy on other satellites. Using publicly accessible documents, the author attempts to piece together the plane’s likely mission, and writes that the X-37B illustrates the United States’ continuing interest in militarizing space and, possibly, weaponizing it in the future. He argues that such an approach inadvertently harms the security of the United States’ own space assets.