Goguryeo–Sui Wars
Sui, having reunified China, invaded Goguryeo four times and failed every time; the campaigns helped bring Sui down.

Yumeto · CC BY-SA 4.0 · Source
When King Yeongyang struck Liaoxi with Malgal troops in 598, Emperor Wen sent 300,000 men, who were driven back by rain and disease. In 612 Emperor Yang came with 1.13 million but could not get past Liaodong Fortress, and his strike force on Pyongyang was destroyed at the Salsu River. Further invasions in 613 and 614 ended in revolt at home, and Sui fell in 618.