Delvestius
Banned
You'll have to be much more specific. "That tasty little peninsula" resisted invasions from the Khitan, Jurchen, Mongols, and Japanese for over 600 years despite little notice before conflicts broke out, along with being severely outnumbered (more than 10:1 in some cases). In addition, Korea had around 200-500 fortresses by the time that the Mongols invaded, and neither the Mongols nor the Japanese managed to overrun everything before the frontline, which cost them severely when guerrilla operations eventually kicked in, as multiple regions prevented the invaders from advancing further after stalling. It's also worth mentioning that it only takes one offer of negotiations from the Korean side to cease operations, which occurred after each campaign IOTL, as there's no point in attacking continously when a country remains militarily defensive and openly offers resources in the form of tribute. If even one fortress resists, the invaders can also be severely affected, as the Mongols withdraw from Kusong (first invasion) after sieging it for 30 days, and withdrew entirely in the second campaign when a Korean monk assassinated a commander with a single arrow, to give specific examples, despite that the Koreans were severely outnumbered throughout the 30-year war, suggesting that the Mongols were unwilling to press on further due to severely depleted resources and morale.
Testaments to the tenacity of Koreans, surely, though sadly for the this Korea, tenacity alone will not see them through. We have:
1. A smaller Korean force ITTL to begin with, say 150-200,000.
2. A Larger Mongol force, say 600-750,000 (The rest of which laying siege in China while some regional governors contemplate surrender).
3. aided by a force numbering 100-150,000 and a navy more than able to contend with the Koreans.
EDIT: In all the campaigns but the first it seems like the Koreans got the worst end of the shtick.