Notes:

OP's Context:

During the occupation of Manchuria and during the initial stages of the Second Sino Japanese War, the IJA made some not-very-subtle forrays into Mongolia, which was a Soviet Satellite at the time. The Mongolians fought back on the same stock of horses that their ancestors once used to terrorize Eurasia. They used modern rifles instead of bows, but their most effective weapon turned out to be the cavalry sabre. In one particular battle, a hundred Mongol horsemen ambushed a Japanese regiment on the march, killing 11 of them before the Japanese were able to set up a heavy machinegun. 23 of the ambushers were gunned down, including their commander, but the survivors fought so ferociously that the Japanese were thrown into a disorderly rout.

After killing their enemies, the Mongols would carry their bodies back across the border to Manchuria as a sign of respect. Japan was trying to portray itself as Asia's master race at the time, and this was absolutely horrible for their PR. They made a couple of half-hearted appeals to the Soviet Embassy to get the attacks to stop, to which the Soviets replied amiably (but forcibly) "the Japanese could avoid such difficulties if they returned to Japan."

This led to an inevitable escalation, and the Mongolians began to rely on the Red Army for help (with dealing with the Japanese tanks, in particular). This campaign saw the meteoric rise of then-Corps Commander Georgy Zhukov as a field commander and culminated with the Battle of Khalkhin Gol, which effectively ended any notions of western Japanese expansion into the Soviet Union.