Well the US gave hunters free gun and ammo and paid them for bison pelts and skulls. At the same time they build the railway so more people can go west. Then they send in their cavalry to protect the hunters. Needless to day the plain indians got hell.

I remember Wudi building a fortified city right on the steppes and encourage (even force) people to move there. Well once the people start to settle down and farm the damn place, the plains are no more. Plus the troops are there to protect the settlers (like in the US-Indian war) so the XiongNu couldn't choose their battleground and could do little but watch as a large piece of their pasture got taken over by farmland.