In the Late Intermediate Period (ca. A.D. 1000-1450), people in many parts of the Andean highlands moved away from rich agricultural lands to settle in defensive sites high on hills and ridges, frequently building hilltop forts known as pukaras in Quechua and Aymara. This settlement shift indicates a concern with warfare not equaled at any other time in the archaeological sequence. While the traditional assumption is that warfare in the Late Intermediate Period resulted directly from the collapse of the Middle Horizon polities of Wari and Tiwanaku around A.D. 1000, radiocarbon dates presented here from occupation and wall-building events at pukaras in the northern Titicaca Basin indicate these hillforts did not become common until late in the Late Intermediate Period, after approximately A.D. 1300. Alternative explanations for this late escalation of warfare are evaluated, especially climate change. On a local scale, the shifting nature of pukara occupation indicates cycles of defense, abandonment, reoccupation, and wall building within a broader context of elevated hostilities that lasted for the rest of the Late Intermediate Period and beyond.