America begins the new year embroiled in two of the longest wars in our history. But so far, the public has not directly borne their costs, which have been deferred to the future or limited to the members of the all-volunteer military. The illusion that our wars are cost-free has reduced the political risks of engaging in military action overseas. But as Princeton professor Julian Zelizer argues in his new history of national security politics, Arsenal of Democracy, it has also made the maintenance of U.S. primacy unsustainable.