Ying Zheng grabbed 36,000 handpicked cavalry and sped south on a mountainous road in order to reassert imperial authority.
Arriving at lower Gansu, Ying Zheng found the Qinglin mountain pass held against him by a strong Tibetan force.
They commanded Ying Zheng to halt while they decided whether or not to admit him.
Unwilling to sit by while this recalcitrant vassal blocked his path, Ying Zheng had his men carve steps into the Western side of mount Qinglin, rode his entire force over, and came up on the Tibetans from behind.
(The Tibetans were actually not vassals Ying Zheng but trading partners and were afraid Ying Zheng came to conquer them and Ying Zheng was too lazy to an explanation to people he considered below him.)
The Tibetans quickly acknowledged Ying Zheng as their overlord after that and moved on unhindered.
(I skipped a lot of information but I will let you fill in the gaps.)
