They've popped one off.
"The nuclear test is a historic event that brought happiness to the [sic] our military and people," the DPRK's official Korean Central News Agency said.
OPLAN 5027 is the US-ROK Combined Forces Command basic warplan. Under Operations Plan 5027 (CINCUNC/CFC OPLAN 5027), the United States plans to provide units to reinforce the Republic of Korea in the event of external armed attack. These units and their estimated arrival dates are listed in the Time Phased Force Deployment List (TPFDL), Appendix 6, to Annex A to CINCUNC/CFC OPLAN 5027. The TPFDL is updated biennially through U.S./ROK agreements. CINCUNC/CFC OPLAN 5027 is distributed with a SECRET-U.S./ROK classification.
Pyongyang can credibly threaten the prompt destruction of Seoul with conventional arms alone. The North Korean military could also establish a shallow foothold across the DMZ. However, the DPRK's ability to sustain these offensive operations, or advance its forces further to the south, is questionable. South Korean and American air forces could quickly establish air supremacy and destroy North Korean ground forces. The ensuing buildup of US forces in Korea could reverse any remaining North Korean advances into the South, and unlease offensive operations into the North. North Korea does not require long-range missiles with nuclear, chemical, or biological warheads to devastate Seoul or to make a land grab across the DMZ. Such weapons are needed to deter or defeat an American counteroffensive into North Korea.
Pyongyang has the ability to start a new Korean War, but not to survive one.
North Korea has about 500 long-range artillery tubes within range of Seoul, double the levels of a the mid-1990s. Seoul is within range of the 170mm Koksan gun and two hundred 240mm multiple-rocket launchers. The proximity of these long-range systems to the Demilitarized Zone threatens all of Seoul with devastating attacks. Most of the rest of North Korea's artillery pieces are old and have limited range. North Korea fields an artillery force of over 12,000 self-propelled and towed weapon systems. Without moving any artillery pieces, the North could sustain up to 500,000 rounds an hour against Combined Forces Command defenses for several hours.
North Korea's short-term blitzkrieg strategy envisions a successful surprise attack in the early phase of the war to occupy some or all of South Korea before the arrival of US reinforcements on the Korean Peninsula. North Korean ground forces, totaling some 1 million soldiers, are composed of some 170 divisions and brigades including infantry, artillery, tank, mechanized and special operation forces. Of the total, about 60 divisions and brigades are deployed south of the Pyongyang-Wonsan line. North Korea has deployed more than half of its key forces in forward bases near the border. Seventy percent of their active force, to include 700,000 troops, 8,000 artillery systems, and 2,000 tanks, is garrisoned within 100 miles of the Demilitarized Zone. Much of this force is protected by underground facilities, including over four thousand underground facilities in the forward area alone. From their current locations these forces can attack with minimal preparations. This means a surprise attack on South Korea is possible at any time without a prior redeployment of its units.
The North Korean navy has also deployed 430 surface combatants and about 60 percent of some 90 submarine combat vessels near the front line in forward bases. With about 40 percent of its 790 fighter planes deployed near the front line, the North Korean air force could launch a surprise attack on any part of South Korea within a short period of time.
The Democratic People's Republic of Korea possesses larger forces than Iraq, and they are already deployed along South Korea's border. A war could explode after a warning of only a few hours or days, not weeks. Unlike in the Persian Gulf, this attack would be prosecuted along a narrow peninsula on mountainous terrain. It would probably be accompanied by massed artillery fire, commando raids, and chemical weapons. Initially, the primary battlefield would be only about 125 kilometers wide and 100 kilometers deep. The DPRK attack would be conducted against well-prepared ROK forces in fortified positions and against larger US forces than in the Persian Gulf. Most probably, the DPRK attack would aim at seizing nearby Seoul by advancing down the Kaesong-Munsan, Kumwa, and Chorwon corridors. If successful, North Korean forces might also try to conquer the entire peninsula before large US reinforcements arrive.
The South Barrier Fence is the Southern part of the DMZ. The South Koreans have a series of Defensive lines that cross the entire peninsula, but with the exception of the South Barrier Fence, they aren't connected completely across the peninsula. They are designed to withstand an attack and allow a minimum force to hold a line while reinforcement/counter attack forces are assembled and sent to destroy any penetrations.
The basic goal of a North Korean southern offensive is destruction of allied defenses either before South Korea can fully mobilize its national power or before significant reinforcement from the United States can arrive and be deployed. The primary objective of North Korea's military strategy is to reunify the Korean Peninsula under North Korean control within 30 days of beginning hostilities. A secondary objective is the defense of North Korea.
To accomplish these ambitious objectives, North Korea envisions fighting a two-front war. The first front, consisting of conventional forces, is tasked with breaking through defending forces along the DMZ, destroying defending CFC forces, and advancing rapidly down the entire peninsula. This operation will be coordinated closely with the opening of a second front consisting of SOF units conducting raids and disruptive attacks in CFC's rear.
The DPRK offensive against the ROK will consist of three phases. The objective of the first phase will be to breach the defenses along the DMZ and destroy the forward deployed forces. The objective of the second phase will be to isolate Seoul and consolidate gains. The objective of the third phase will be to pursue and destroy remaining forces and occupy the remainder of the peninsula.
Approximately forty percent of the South Korean population resides within 40 miles of Seoul. While the terrain north of Seoul is dominated by rice paddies offering limited off-road mobility, the terrain west of Seoul is a wide coastal plan with the main invasion routes to Seoul. North Korean forces attacking Seoul through the Chorwon or Munsan corridors would have to cross the Han or Imjin rivers (while these rivers freeze in the winter, the ice is not strong enough to support heavy armor). The narrow eastern coastal plain is lightly settled and less heavily defended, though mountains make movement of forces from the east coast difficult.
The US plans are based on the belief that the North Koreans would not be successful in consolidating their gains around Seoul and could be pushed back across the DMZ -- though the plans assume the North may break through the DMZ in places. A critical issue is strategic warning of unambiguous signs that North Korea is preparing an attack. The warning time has reportedly been shortened from about ten days to about three days as North Korea has covered its military activities.
The US-ROK defense plan would be shaped not only by the threat but also by the mountainous terrain. Korea is commonly regarded as rugged infantry terrain that invites neither mobile ground warfare nor heavy air bombardment, but North Korea has assembled large armored forces that are critical to exploiting breakthroughs, and these forces would pass down narrow corridors that are potential killing zones for U.S. airpower. A new Korean War would bear little resemblance to the conflict of 195053.
During Phase 1, US-ROK forces would conduct a vigorous forward defense aimed at protecting Seoul. Their campaign would be dominated by combined-arms ground battles waged with infantry, artillery, and armor. US air and naval forces would conduct close air support, interdiction, and deep strike missions. After Phase 1, US-ROK operations in Phase 2 would probably focus on seizing key terrain, inflicting additional casualties on enemy forces, and rebuffing further attacks. Phase 3, to start when the US ground buildup was complete and ROK forces were replenished, would be a powerful counteroffensive aimed at destroying the DPRK's military power. The war plan envisions amphibious assaults into North Korea by US Army and Marines at the narrow waist of North Korea. The entire resources of the US Marine Corps would flow there to establish a beachead, with substantial Army resources quickly conducting over-the-shore operations.