Why did the USSR Refuse to Annex Mongolia? (Short Animated Documentary)

From a channel Lumen includes in full: History Matters answers specific history questions ('Why didn't the Boers unite...?', 'How did Brazil get so big?') in short, tightly argued animated explainers.

A 3-minute video from History Matters's catalogue; the title names its topic.

Video by History Matters, played through YouTube's embedded player.