There were the fiercest fighters in their period and Han almost lost if not for Wudi. He basically emulated their fighting style (light calvary brigades instead of mass infantry), incorporated their horses into the Chinese calvary and crushed them. They won the war with mostly about 40-50 thousand men against XiongNu's over 200,000 (outnumbering by 4-1). After that, part of Xinjiang today belonged to China and the Silk Road is opened to the West.
Yeah, there are traces of the Hun from Xiongnu as well as mongolians/Russians. Basically Kazhakstan, and people around taht region were the XiongNu's.
He had the help of another Caucasian tribe named the Yuezhi who had already been beaten by the Xiongnu.
The Chinese came to know them as the greater and lesser Yuezhi, the greater Yuezi allied with the Han against the Xiongnu.
BTW, there are graveyards in the area that contain only Caucasian skeletons that date all the way up to the 1300'sAD. In The Loulan region, the Mongoloids and Caucasians began to mix around the 100-200BC time frame. Previous to that time, there were no Mongoloid skeletons anywhere in the region.