A couple of nukes, biological weapons, chemical weapons and one of the largest artillery forces in the whole world all massed against Seoul.
http://www.businessinsider.com/heres...h-korea-2016-4
South Korea can win if we just compare forces on paper, but the vast bulk of their military is made up of conscripts whose morale would collapse in the first days of artillery shelling and the complete devastation of Seoul. The North Koreans are far more highly motivated due to all the indoctrination they've been through.
The reason the US would wipe the floor with the North Koreans is that the US military is made up of the most determined well trained soldiers in that world. It's not the tech or numbers that make the US military the most powerful in the world, it's the will of it's soldiers to fight hard.