Dramatic social changes in the early twenty-first century United States led to dramatic shifts and schism in a variety of religious groups. The Mennonite Church USA, a denomination created from the merger of two older bodies, the Mennonite Church and the General Conference Mennonite Church, has wrestled with these same social justice issues, most notably sexuality and gender, from its inception in 2002. This work situates that struggle within the larger religious landscape of the United States. In doing so it traces the history of both the Mennonite Church and the General Conference Mennonite Church, examining the ways in which social conflicts in the twentieth century, such as the civil rights movement and the war in Vietnam, were dealt with by these two groups. In exploring that history, three distinct groups appear in both denominations, two of which were heavily informed by fundamentalism. These differences, which grew over the course of the twentieth century, coupled with distinct differences in polity – the way these denominations were governed – not only explain the varied ways Mennonites engaged with the social issues of the twenty-first century, but also explain the rapid decline of the Mennonite Church USA.