The Indian summer monsoon (ISM) is an important annual climate cycle that has major implications for human well-being, in part because it influences seasonal rainfall in South Asia. As a result, climate researchers have sought to understand the factors that cause the ISM's intensity to vary and how climate change may affect it. Traditionally, researchers have thought that, during the Pleistocene (∼2.6 million years before the present), the ISM weakened during cooler periods as glaciers in the Northern Hemisphere (NH) expanded, and the monsoon strengthened during warmer periods as the ice sheets melted. This paradigm now faces a serious challenge. On page 719 of this issue, An et al. (1) present evidence that—contrary to the conventional view—the ISM often reached its weakest point, and even began to strengthen, before global volumes of glacial ice reached a maximum. They also propose that, during glacial periods, the development of the ISM was driven by cooling in the Southern Hemisphere (SH), highlighting the role of interhemispheric interaction in governing the relationship between the ISM and glacial cycles.