 Complete the following phrase:  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. 14187 BCE 