Key point: The 1.2 million man Korean People’s Army is organized into nineteen corps-sized units, including nine infantry corps, four mechanized corps, one armored corps, one artillery corps, the Pyongyang Defense Command, Missile Guidance Bureau and Light Infantry Instruction Guidance Bureau.

North Korea is just slightly larger than Ohio. To the south it borders South Korea, to the west it borders the Yellow Sea, and to the east it borders the Sea of Japan. To the North it shares an 880 mile border with China and a much smaller one with Russia. The southern border is heavily fortified, with a 2.5 mile demilitarized zone separating the two Koreas. About a tenth of the population resides in the capital, Pyongyang, with the rest primarily residing in cities on both coastlines, often separated by water, hilly or rough terrain.

Any invasion of North Korea would have to take account these geographical realities. The 1.2 million man Korean People’s Army is organized into nineteen corps-sized units, including nine infantry corps, four mechanized corps, one armored corps, one artillery corps, the Pyongyang Defense Command, Missile Guidance Bureau and Light Infantry Instruction Guidance Bureau. More than half of these forces, particularly the mechanized, armor, and artillery forces are located near the DMZ, making an early cross-border assault unattractive.

The Korean War is unique in that a war has already been fought over the same terrain, against the same enemy, in a largely conventional war. Its legacy suggests that if the United States and South Korea wish to invade the North, an amphibious assault would be the opening blow. North Korea has 1,550 miles of coastline, and while not all of it is favorable to amphibious operations there is plenty that is.

One obvious target to kick off the invasion is the coastal city of Wonsan. Located on the country’s eastern coast it is longitudinally opposite the capital Pyongyang with a good road connecting the two cities. The area is home to a North Korean mechanized and infantry corps, making it a less than ideal invasion sector, but Wonsan’s port and nearby airport would provide an essential springboard for an offensive into the heart of the country. A U.S. Marine Corps Marine Expeditionary Force the size of a reinforced brigade, drawn from forces in Okinawa, Hawaii, and California would conduct the operation, reinforced by carrier aircraft from the USS Ronald Reagan.

At the same time U.S. Marines are hitting the beach on the East Coast, South Korean marines would launch an amphibious assault south of Pyongyang and north of the DMZ. Their objective would be to seize Pyongyang itself with the intention of neutralizing the DPRK leadership early on. This assault would be difficult but preceded by South Korea’s “Kill Chain” contingency plan, ideally throwing North Korea’s defenses into chaos.

The capture of Pyongyang would panic DPRK forces along the border who, upon realizing they are cut off from their command would likely march north. U.S. Air Force and Republic of Korea Air Force bombers and strike aircraft flying from Okinawa, Japan, South Korea and Guam would fly interdiction missions, striking those armored and mechanized columns moving to meet their invaders. This will be likely the best opportunity, aside from ground combat, of destroying large numbers of KPA forces. That having been said, detecting North Korean troops on the move in the dense mountainous terrain is a challenging prospect.

As North Korean forces pull away from the DMZ, U.S. and South Korean army forces launch cross-border attacks with the intent of linking up with U.S. and South Korean marines. Both offensives would not be easy but KPA forces, in both cases caught between two substantial ground forces and hounded from the air, could sustain heavy casualties. In the meantime, U.S. and South Korean special forces would launch raids against known chemical, biological, and nuclear facilities—particularly the latter—to prevent their use or disappearance into unknown hands.

Linking the two campaigns on opposing coasts will be difficult and like the 8th Army and X Corps during Korean War I the two campaigns will likely stay separate. Both campaigns will need to speed north to secure the borders with China and Russia, but here they also pose the same danger: that doing so will trigger a Chinese (or Russian) intervention directly against U.S. and South Korean forces. Alternately, Chinese forces may carve out a buffer zone inside North Korea to prevent refugees and elements of the KPA from entering China. Ultimately no one knows what China would do, but the threat of a wider, open-ended war over the entire Asia-Pacific region should give everyone pause.

The onset of war will trigger several races against time. The first and most important from the U.S. perspective is the neutralization of the Kim regime, its long-range missiles and chemical weapons. Next, although conventional victory is assured, South Korea and the United States must rapidly defeat the KPA in order to disrupt any effort to convert it into an insurgent force like Iraq’s Feyadeen Saddam. Third, both countries must quickly reach the cities to prevent a humanitarian disaster. As the capital rapidly falls into enemy hands, North Korea’s economy and food distribution system, already weak, will rapidly fold, leaving millions without food.

A ground war with North Korea would be an extremely complex operation with considerable military and civilian casualties on both sides. In fact, the ground war would be a mere slice of a multidomain conflict likely to involve missile attacks against Guam, Japan and South Korea, convoying, minesweeping, and other sea-based operations involving U.S., South Korean and Japanese naval forces, and even the readying of U.S. strategic forces. Unlike recent operations U.S. forces would face considerable risk in large numbers, operating against a numerically superior foe. Although American technological advantages, particularly in the areas of communications, mobility, and firepower would allow U.S. and South Korean forces to ultimately prevail, there would be little room for error.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009, he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami. This article first appeared several years ago.

Image: Reuters.