The Han–Xiongnu War, also known as the Sino–Xiongnu War, was a series of military conflicts fought over two centuries between the Chinese Han Empire and the nomadic Xiongnu confederation, although extended conflicts can be traced back as early as 200 BC and ahead as late as 188 AD. The Chinese civilization initially clashed with nomadic tribes that would later become the Xiongnu confederation during the Warring States period, and various northern states built elongated fortifications to defend against raids from the Mongolian Plateau. Wikipedia