In June 1938 China's Nationalist government breached a major Yellow River dike in a drastic attempt to use flooding to slow the Japanese invasion. The strategic breach caused the Yellow River to abandon the northern course it had followed since 1855, and its new southeastern course led to eight years of catastrophic flooding. After World War II, the Nationalists, with extensive aid from the United Nations Relief and Rehabilitation Administration (UNRRA), aimed to close the breach and divert the river back to its pre-1938 course. However, the Chinese Communists had taken control of much of that course, and local interests there opposed the plan to bring back the river. The Yellow River diversion project thus became intensely politicized. This article examines how the diversion plan became embroiled in the Chinese Civil War of 1946–49, how the river's return to its northern course in 1947 impacted communities in its path, and how the Communists and Nationalists imagined the river and made different tactical and rhetorical uses of it during the war. I find that the campaign to reroute the river was complicated not only by the civil war but also by tension between local and national interests within the Communist Party, and that UNRRA's attempts to mediate between the Nationalists and Communists at times put the organization at odds with both parties. Moreover, in 1946 and 1947 the intense struggle to tame, make strategic use of, or cross the Yellow River became an important metaphor for the battle to control China.