This dissertation examines the origins and evolution of the trained reserve—a 19th-century military institution that most states continue to depend upon for national defense. In doing so, it argues that the rising complexity, speed, and lethality of warfare over the last century challenged the viability of industrial era reservist personnel and training policies, which many states retain today. Such policies have largely proven incapable of providing reservists with the time and resources to keep up with changing technical and tactical character of warfare, as evidenced by the poor performance of many reservists in the conflicts of the past century. However, armies preparing for large-scale combat operations generally could not reduce their reliance on reservists because of a combination of political, military, and socio-economic constraints and compulsions. Namely, they lacked the financial means, military resources, and, in some cases, the political capital to rely solely on full-time soldiers who often are better resourced to keep up with the rising complexity of modern warfare compared to part-time reservists.Thus, as this dissertation reveals using archival sources, recently declassified intelligence reports, and a host of other printed primary sources, armies confronted a dilemma regarding reserve policy over the past century. They could, as the U.S. Army has done since the 1970s, increase reserve training standards, but risk incurring higher overhead costs or sparking a political backlash from reservists who often resisted efforts to increase their service obligations. Alternatively, they could reduce their dependency on reservists, as the Israel Defense Forces (IDF) have done since the late 1980s. But doing this threatened to overstretch active duty units who, as this dissertation shows, generally cannot fight protracted or large-scale wars without reservists. In short, this dissertation shows how the dynamics of reserve policymaking and the combat performance of reservists can—and often had—significant political, strategic, operational, and tactical effects throughout recent history.