In my story, a portal links the modern world to a medieval fantasy world. This is sort of similar to Gate: Thus the JSDF Fought There. The North Korean military decides to go through their portal and acquire more land and resources for Kim Jong-Un.
The first polity The Korean People's Army encounters is the Orc Federation. The Orc Federation is a massive group of pastoralists and villagers who often raid and demand tribute from nearby communities. The Orc Federation is heavily inspired by the Timurid Empire. The geography and size of their empire is equal to Earth's Central Asia.
The orcs only have access to 15th Century technology which is obviously no match for the weapons the North Koreans have at their disposal. The orcs also completely lack any magic. However, the orcs do possess some advantages over not just the North Koreans but humans in general. The first advantage is that they are defending and are really motivated and know the layout of their land better but that goes without saying and that advantage is present in most wars. What makes orcs special is they reproduce fast. An orc woman has a gestation period of only 6 months and generally gives births to twins. Orcs also go through puberty around age 7 and have a life expectancy of 50 years. As a result, the orcs have a large and young population compared to similar empires, 500 million people to be exact which is much larger than North Korea's population or any modern country for that matter excluding China and India.
The orcs second major advantage is their physical attributes. The Orcs weigh 200 kilograms on average and are around 2.5 meters tall. Despite their size, the Orcs have the speed of a cheetah, the strength of a gorilla, and the durability of an African elephant. As a result, an orc in close quarters can lay waste to many human opponents with ease.
The final advantage of orcs is their sexual dimorphism. Orc women are just as big and strong as the men so they can participate in combat just as well. This potentially doubles the size of the orcish forces before they run into logistical and demographic problems.
Even a herd of fast elephants won't last long in a battle against modern artillery and naval ships and tanks and aircraft. That is why after the first few major losses, the orcs stop fighting fair and no longer attempt or participate in set-piece battles. The orcish military takes off their uniforms and starts fading into the shadows and dressing like civilians. The orcs might not be able to do much to the tank or fighter jet in a fight but they can still kill even modern infantry with ease. Whenever soldiers go on patrol, especially at night, the orcs ambush the ones who stray too far from the base; generally killing them before they even realize what's going on. The orcs start sneaking behind enemy lines and start raiding lightly-defended outposts and attacking enemy supply lines. The orcs start launching insurrections against the North Koreans who are guarding occupied towns and villages. The orcs start sneaking onto ships at night and begin killing the crew. The speed of the orcs combined with their toughness means assault rifles aren't very effective against them. If the orcs can get to the humans before they can deploy heavier weaponry or call for backup, they could perform a lot of successful attacks.
Soon, it is becoming less and less safe for squads or even platoons of infantry to venture into orc territory. The soldiers are mostly safe in their bases and vehicles but they have to leave their forts and vehicles at some point. Occupying the Orc Federation would get more and more expensive, especially when the military has to be far more mechanized against a foe than you would expect against a foe with mostly medieval technology. Later on, NATO starts arming the orcs with rocket launchers to take out North Korean vehicles.
So the question is could medieval, non-magic superhumans win an unconventional conflict against a modern military like North Korea? A lot of the advantages of the modern military industrial complex are nullified in guerilla warfare, which is why guerillas have waged war against the USA and Soviet Union for so long.