For decades after the Korean War, the military focus on the peninsula was on three invasion routes from north to south, dictated by the mountainous terrain. The Democratic People's Republic of Korea, as North Korea calls itself, dug tunnels under the demilitarised zone, from which commando raids could begin an invasion to overrun the south to the city of Busan in as few as three days.
Today that possibility is essentially dead. The DPRK's military is still almost twice the size of its counterpart in the Republic of Korea (South Korea), but is hopelessly outclassed. DPRK forces would probably run out of fuel for their ageing air force within days.
But these deficiencies have not stopped Pyongyang from goading its southern neighbour. In quick succession in the last two weeks, the DPRK renounced a post-war armistice; threatened to attack Seoul and a South Korean island in the Yellow Sea; announced "command readiness" to launch missile strikes against the US; declared a "simmering nuclear war"; and formally cut the last military hotline with the south.
Many analysts say this is one of the most dangerous times on the Korean Peninsula since the war.
But North Korean belligerence is nothing new; threats to reduce Seoul to a "sea of ashes" have become routine. Rather, the heightened risk of escalation this time is tied to events in the south.
The new administration of President Park Geun-hye has responded with an uncharacteristically tough line in recent weeks, with one South Korean general promising that a DPRK attack would be met with a military response targeting the "command leadership", which would include the DPRK's young leader, Kim Jong-un. The United States has backed its ally, signing a defence pact with South Korea in case of a provocation and strengthening counter-missile defences, including along the US west coast.
In part, these statements are an exercise in symbolism, much like the DPRK's habitual "sea of ashes" rhetoric. But a credible threat of retaliation does indeed change the complexion of a decades-old conflict.
It has always been a mistake to discount the DPRK's threats as irrational, even when they are improbable. Indeed, the 1953 armistice that was cancelled this month was hardly worth the name. In attacks in March and November 2010, the DPRK torpedoed an ROK corvette, the Cheonan, killing 46 sailors and launched artillery and missile strikes at Yeonpyeong Island, killing four.
It's been a familiar pattern for the south: in 1968 DPRK commandos threatened the presidential house; in 1983 an assassination attempt on South Korea's president, Chun Doo-hwan, killed 14 Koreans in Rangoon, then the capital of Burma; in 1987 terrorists downed a Korean Airlines plane.
There are few countries - and probably none that have military superiority - that would countenance such attacks with the ROK's characteristic restraint. The diplomatic deadlock over the nuclear-weapons programme in the past decade has distracted from tangible military realities: the DPRK has between 10,000 and 12,000 artillery pieces within range of Seoul, and a chemical-weapons capability.
There are reasons why that calculation might be changing. The DPRK's dream of a land invasion has been crippled by its lagging economy, the US missile shield is more robust than it has ever been, and overwhelming air superiority mitigates, if not neutralises, the artillery threat. The DPRK has lost ground in the conventional-warfare balance faster than it has made progress on a nuclear weapon.
The other passive deterrent, however, is the prospect of the DPRK's collapse. Few South Koreans look forward to reunification and the prospect of lifting 24 million compatriots out of their moribund economy, much less dealing with decades of mutual antagonism.
Conciliation and a so-called "sunshine policy", which was meant to improve relations through aid and outright bribery, have been an abject failure. If the regime is to survive, the DPRK's policy of "military first" cannot be abandoned.
The most recent nuclear-weapons test, in February, appeared to be the most successful to date, although the threat to launch missiles at the US west coast is probably years away from being serious. The DPRK's ability to build a reliable nuclear weapon, with its dwindling fissile stocks, and to miniaturise it to fit in a missile payload, is also in question.
There are, however, still plenty of options for the north. The Cheonan was sunk along the Northern Limit Line - the Yellow Sea maritime border that the DPRK disputes - in an area where provocations have occurred before. Baengnyeong island, which DPRK propaganda threatened last week to "wipe out", and Yeonpyeong, where residents are doing bomb-shelter drills, are both in the Yellow Sea and both closer to the mainland north than to the south.
It is hard to imagine that the ROK and the US would back away from another provocation on the scale of the Cheonan or the Yeonpyeong bombardment. Another attack along the DMZ, such as the 2008 shooting of a South Korean tourist, would demand a more measured response.
Perhaps most likely in the near term is a provocation in cyberspace. DPRK hackers are naturally suspected in last week's denial-of-service attacks against South Korean broadcasters and banks. Last year, the DPRK jammed GPS signals that disrupted hundreds of flights, which South Korean officials said rose to the level of a terrorist attack.
In Pyongyang, leaders will be interested in avoiding a war that they cannot win. Their options are limited, but their arsenal still allows them to test the ROK's new determination.
Jeremy Walden-Schertz is former Comment Editor at The National and a freelance journalist specialising in East Asian politics
On Twitter: @JWaldenSchertz