[Why] Goguryeo in the northeast had grown steadily stronger, and its geopolitical ambition seriously threatened Sui's traditional tributary defenses over Liaodong and the Korean peninsula. Emperor Yang resolved to mobilize national military force and finish the matter in one push. [What] Emperor Yang of Sui launched three successive ultra-large campaigns, mobilizing over a million troops to march on Liaodong. But due to command confusion and poor tactics, the Sui armies repeatedly suffered devastating annihilations before strong city walls. [Who] Emperor Yang of Sui was reckless in strategic command and despite generals' protests insisted on his folly. Thousands upon thousands of elite Sui troops died in the icy wilderness of Liaodong as the brutal sacrifice to his obsession. [How] Three fruitless and ruinous wars exhausted the last vital core economy and commoner energy of the new empire. They ignited a nationwide peasant uprising that swept the country and led to the Sui's plummet into ruin.
Why
The historic event of Three Campaigns against Goguryeo represents a key developmental peak of the Huaxia dynastic system. Sparking nationwide peasant rebellions that led to the collapse of the Sui. By establishing this moral or administrative benchmark, it continues to shape the structural and philosophical fabric of ancient Chinese statecraft.