This dissertation is a labor history of the United States army in the nineteenth and early twentieth centuries. It argues that soldiers constituted a partially-unfree labor force essential to the advance of U.S. imperialism. The project draws on government reports, military records, court-martial testimonies, memoirs, letters, and newspapers. It examines the U.S.’s military labor regime in pre-statehood California, the Reconstruction South, the trans-Mississippi West, and the southern Philippines. Military labor, often performed under duress, allowed the United States to extend its authority across North America and around the world. Soldiers built roads and telegraph lines; mapped territory; supervised elections; assisted railroads and other private companies; and governed subject populations. This state-sanctioned labor regime relied on coercive and violent practices. Soldiers were paid less and enjoyed fewer rights and protections than their civilian counterparts. They were subject to physical punishments and humiliations for various infractions. In the most telling difference between military and “free” labor, soldiers could not leave their assignments even one day short of their enlistment period; “quitting” the army was criminalized as desertion. As the United States extended its influence over new parts of the world, its military labor regime also expanded to include local populations. This study examines the army’s attempts to exploit the labor of soldiers and indigenous populations as well as resistance to these efforts. This project reframes the army’s contributions to U.S. imperialism. Beyond the battlefield, soldiers worked, often under duress, on behalf of an ambitious and expansionary state. By focusing on the army’s labor regime, it demonstrates the centrality of both the military and unfree labor to the construction of an American empire in the nineteenth and early twentieth centuries.