Today, much of the talk about North Korea is in regard to its nuclear weapons program and the concern that they could reach Japan, or even the U.S. with ballistic missiles. But one aspect of North Korea's military prowess is that its army could threaten South Korea's relatively weak one.
NorKor has the 4th largest standing army in the world with 1.2 million members. Even larger than Russia's; and just 300,000 shy of all of our U.S. forces. However, unlike the U.S. military which is stretched all over the world, NorKor's army is singularly focused on South Korea. More importantly, NorKor has the world's largest paramilitary reservist force of nearly 6 million that could easily supplement any conflict with South Korea. 25% of its entire population!
But, the real problem is that South Korea's army stands at just 500,000. Add to this the 28,500 American soldiers in the "South" and, as anyone can see, NorKor is a very intimidating force to deal with. In addition, the "North" has steadily built up its conventional weapons while the "South" focuses less on its military and more on economics. In other words, without additional and extreme intervention by the U.S., South Korea would be a sitting duck in a war with North Korea.
North Korean Army: https://en.wikipedia.org/wiki/Korean_People%27s_Army#People.27s_Army_Air_Force_and_Air_Defence_Forces
List: Largest Standing Armies in the World: https://www.google.com/search?safe=off&q=largest+standing+armies&spell=1&sa=X&ved=0ahUKEwi6rvW9g7vTAhVDwGMKHXkUDLgQvwUIIigA
United States forces in South Korea: https://en.wikipedia.org/wiki/United_States_Forces_Korea
South Korean Army: https://en.wikipedia.org/wiki/Republic_of_Korea_Army