Sui Yangdi
At the end of the 6th century, wars between the Sui dynasty and Goguryeo. Sui dynasty launched a total of 4 invasions, in 598, 612, 613 and 614, but all were defeated. Notably, in 612, Emperor Yang of Sui mobilized an army of up to a million men to attack Goguryeo but failed miserably. The wars with Goguryeo severely weakened the Sui dynasty, further destabilization of the Sui dynasty government, hastening its collapse.