In 209 BC, three years before the founding of Han China, the Xiongnu were brought together in a powerful confederation under a new chanyu, Modu Chanyu. This new political unity transformed them into a more formidable state by enabling formation of larger armies and the ability to exercise better strategic coordination. After their previous rivals, the Yuezhi, migrated into Central Asia during the 2nd century BC, the Xiongnu became a dominant power on the steppes of north-east Central Asia, centered on an area known later as Mongolia. The Xiongnu were also active in areas now part of Siberia, Inner Mongolia, Gansu and Xinjiang. Their relations with adjacent Chinese dynasties to the south east were complex, with repeated periods of conflict and intrigue, alternating with exchanges of tribute, trade, and marriage treaties (heqin).