The Han dynasty ruled in an era of Chinese cultural consolidation, political experimentation, relative economic prosperity and maturity, and great technological advances. There was unprecedented territorial expansion and exploration initiated by struggles with non-Chinese peoples, especially the nomadic Xiongnu of the Eurasian Steppe. The Han emperors were initially forced to acknowledge the rival Xiongnu Chanyus as their equals, yet in reality the Han was an inferior partner in a tributary and royal marriage alliance known as heqin. This agreement was broken when Emperor Wu of Han (r. 141–87 BCE) launched a series of military campaigns which eventually caused the fissure of the Xiongnu Federation and redefined the borders of China. The Han realm was expanded into the Hexi Corridor of modern Gansu province, the Tarim Basin of modern Xinjiang, modern Yunnan and Hainan, modern northern Vietnam, modern North Korea, and southern Outer Mongolia. The Han court established trade and tributary relations with rulers as far west as the Arsacids, to whose court at Ctesiphon in Mesopotamia the Han monarchs sent envoys. Buddhism first entered China during the Han, spread by missionaries from Parthia and the Kushan Empire of northern India and Central Asia.