This book examines the partnerships and power struggles between American missionaries and Korean Protestant leaders in both nations from the late 19th century to the aftermath of the Korean War. Yoo analyzes American and Korean sources, including a plethora of unpublished archival materials, to uncover the complicated histories of cooperation and contestation behind the evolving relationships between Americans and Koreans at the same time the majority of the world Christian population shifted from the Global North to the Global South. American and Korean Protestants cultivated deep bonds with one another, but they also clashed over essential matters of ecclesial authority, cultural difference, geopolitics, and women's leadership. This multifaceted approach - incorporating the perspectives of missionaries, migrants, ministers, diplomats, and interracial couples - casts new light on American and Korean Christianities and captures American and Korean Protestants mutually engaged in a global movement that helped give birth to new Christian traditions in Korea, created new transnational religious and humanitarian partnerships such as the World Vision organization, and transformed global Christian traditions ranging from Pentecostalism to Presbyterianism.