Still, given the KPA's logistic weakness and inability to sustain battlefield operations, analysts expect an offensive lasting only three days to one week, after which Pyongyang could negotiate from a position of strength.
Meanwhile, could South Korean forces hold long enough for U.S. troops to massively reinforce? Could U.S. forces operate effectively with their bases in Korea -- and possibly Japan, Okinawa and Guam-- under attack by KPA commandos and missiles? These are the imponderables.
Commandos would provide the KPA's spearhead, infiltrating by air, sea and probably under civilian cover to assault South Korean infrastructure and U.S. bases, degrading Seoul's command and communications capabilities and stemming U.S. reinforcements, said Kim of Korea University. Chaos would likely be increased by electronic jamming measures and cyber attacks. Meanwhile, KPA artillery could fire thousands of shells in their opening barrage, Kim estimated.
Still, questions hang over the KPA's war-worthiness. During Pyongyang parades, goose-stepping battalions display the world's finest close-order drill, but under U.S. aerial bombardment, might Kim's legions -- like Saddam Hussein's -- crack?
It seems unlikely. When North Korean troops have engaged -- notably in Yellow Sea clashes in 1999, 2002 and 2010, and in commando raids in 1968 and 1996 -- they have proven skilled and motivated.
But neither special forces nor artillery are war winners alone: They cannot seize and hold ground. The KPA's biggest weakness is the vulnerability of its main force units once they begin to maneuver.
The U.S. and South Korea could fight a three-dimensional battle: KPA infantry and armored units would be pummeled by 24-7 U.S. aerial bombardment; its forces would also be vulnerable to heli-borne envelopment; and, because Korea is a peninsula, the North could be flanked by sea in amphibious operations.
Still, if the KPA ran the 30-mile gauntlet from the border and broke into Seoul, a city vaster than Stalingrad, it would be easy to cut off but difficult to evict. Close combat among Korea's hills and streets could prove murderous.
"They're not Saddam's army, they're likely to fight like the Japanese in the Pacific," said Pinkston, referring to Japan's last-ditch island stands of 1944-5. "They would be paranoid about what would happen if they surrendered."
Destroying North Korean artillery shelling Seoul -- much of it emplaced in tunnels that have been dug over decades -- would be another stern task. Kim noted that U.S. "bunker buster" bombs used in Iraq were originally designed for use against North Korea.
Seoul and Washington possess precision-guided munitions. Bombs or missiles bursting in bunker entrances could bury KPA artillery and air force units, analysts say. But the South Korean capital would likely take a severe pounding -- possibly with unconventional weapons.
Last March, Thurman said: "If North Korea employs biological weapons, it could use highly pathogenic agents such as anthrax or the plague. In the densely populated urban terrain of the ROK, this represents a tremendous psychological weapon."
A marine or airborne landing to its rear are options to take out North Korea's gun line; the question is how much damage Seoul would suffer before such operations could be launched. KPA missiles are an additional threat: As coalition forces discovered in Gulf War I, finding and destroying mobile launchers is tremendously difficult.
Yet with U.S. air power constantly degrading KPA units, communications, headquarters and logistics nationwide, experts see no way for Pyongyang to win a sustained war. If South Korea and the U.S. attack into the North, the wild card is Beijing, with whom Pyongyang has a mutual defense treaty.
Northern Korea guards China's northeast: throughout history, a strategic flank. In 1950, with North Korea largely overrun by U.N. forces, Beijing intervened, saving the state from extinction. Pundits say Beijing would not support a Pyongyang offensive, but would defend her -- suggesting Kim's regime could survive a war, as his grandfather did.
"China will support North Korea, but only on North Korean territory," said Choi Ji-wook, head North Korea researcher at Seoul's Korea Institute of National Unification. "They will not support a North Korean army attacking South Korean territory."
Washington wants a tougher Chinese stance toward North Korea, but it is unclear whether Beijing's six-decade policy of support has altered significantly.
While supporting a vote to impose tougher sanctions on North Korea after its nuclear test, China recently criticized an announcement from the U.S. that it was beefing up defense systems along the U.S. West Coast.
"Bolstering missile defenses will only intensify antagonism, and it doesn't help to solve the issue," Hong Lei, a spokesman for the Chinese Ministry of Foreign Affairs, said at a regular news briefing in Beijing.
And regardless of the Chinese role, Kim Jong Un, North Korea's young leader, possesses a doomsday option: The nuclear button.
Currently, Pyongyang is not believed to have a missile-mounted nuclear warhead, but it may in years to come. Experts believe the North has rockets able to hit Japan or South Korea with air, land or sea-delivered nuclear devices or dirty bombs. If Kim detonated a nuclear device, it would guarantee apocalyptic retaliation and war crimes trials for any regime survivors -- but if all looked lost, that possibility stands.