Amid a cloud of ocher dust, the South Korean Army colonel vaults out of an open jeep, thrusting a swagger stick, aviator's sunglasses and a fur-trimmed muffler toward his driver. Over his shoulder the pillbox-encrusted ridges of Communist North Korea are faintly visible under the gunmetal skies.

A tall, rugged man, he commands a remote garrison here in an area called the Iron Triangle on a central part of the 2 1/2-mile-wide, 155-mile-long demilitarized zone that has separated the two Koreas for 29 years. Across it today hostile armies totaling 1.3 million men shadowbox.

Reflecting sentiments common to front-line duty here, the soldier, who does not want his name used, tells a rare civilian visitor that he is ready "to defeat the enemy at the first sign of war," something he believes could come at any time. His role is to kill potential invaders because, he says through an interpreter, "There will be no lasting peace as long as Communists exist in this world, especially North Korean ones."

In Seoul, the increasingly affluent capital of South Korea, North Korea-watchers stress that they are not predicting that an attack is imminent or even highly probable.

At the same time, North Korea's burgeoning military buildup worries South Korean and American analysts who say it could help spark another war should Pyongyang decide that the time is ripe to attempt to reunify the peninsula by force.

The prospects of peaceful reunification, the subject of a now broken North-South dialogue, appear remote. And while the paltry flow of information from Pyongyang's cloistered government means attempts to assess accurately what the North may do are little more than sophisticated guesswork, some of the signs are disturbing.

According to U.S. intelligence estimates, the North routinely spends as much as 20 percent of its gross national product on its military. In 10 years, the total of its armed forces personnel has doubled to roughly 800,000; the South has slightly less than 600,000 men under arms, including the 39,000 U.S. troops stationed here.

North Korea is believed to have bolstered its strength in forward areas along the DMZ where artillery emplacements and other military installations are heavily fortified or dug underground. Its special commando force of 100,000 men, one of the world's largest, has expanded roughly tenfold since the late 1960s and is trained in the quick-strike tactics that would be needed to mount an invasion over central Korea's mountainous spine.

"The simple fact," said a senior diplomatic source in Seoul, "is that the North outguns the South, 2 to 1," in the tanks, warships, mobile artillery and other offensive hardware that blitzkreig attacks are made of. With the DMZ only 26 miles from Seoul, roughly the distance between Dulles Airport and downtown Washington, "there is ample cause for concern."

Inside the Iron Triangle, a prewar complex of industrial towns at the heart of the peninsula, the mutual hostilities and intermittent violence that have split North and South are part of the daily round. Along the triangle's base, formed by the cities of Chorwon and Kimwha in South Korea, farmers till trellised paddy fields amid windswept granite bluffs, dun-colored hollows and a web of concertina wire, mine fields and concrete tank traps.

It was through the triangle and down the narrow corridor of its riverine plateau that the main invasion force of North Korean tanks roared south in the early morning of June 25, 1950. Today, its apex, the city of Pyongyang, lies in North Korea as a result of the 1953 armistice that ended the Korean War with 53,000 Americans and more than 2 million Koreans dead and created the DMZ. Since then, border clashes and other incidents have killed dozens more American soldiers and more than a thousand Koreans.

The area is the site of one of three subterranean tunnels, blasted out of solid rock, that have been discovered under the DMZ in the past 10 years. According to South Korean and U.S. authorities, the tunnels were designed and dug by the North as possible new invasion or infiltration routes. Officials contend that there are at least another dozen or so tunnels under construction beneath the DMZ, indicated by regular digging noises and the sound of underground blasts.

In assessing the potential threat posed by North Korea, an American diplomatic analyst said, "the next four or five years may be a critical period" militarily. The United States and South Korea are moving to close the gap by upgrading their combined capabilities. Faced with a narrowing strategic superiority, Pyongyang, the argument goes, increasingly may be tempted to move militarily.

South Korea spends 6 percent of GNP, or roughly one-third of its yearly national budget, on its military. And barring any serious downturn, the South's economy promises to continue to outstrip and outpace that of the North.

The South has 38 million people and a $58 billion economy, or about twice the population and five times the North's economic scale.

Competition for international acceptance, meanwhile, has escalated. To the apparent chagrin of North Korean President Kim Il Sung, Seoul is designated to host the 1988 Summer Olympics and, if all goes smoothly, South Korea will become only the second Asian country after Japan to host the Summer Games. Recent diplomatic efforts to drum up support by both sides in Southeast Asia appear tipped in the South's favor because of its greater access to world markets, technology and its strong exports.

"You can't really calculate any equation," Gen. John A. Wickham Jr., former commander of the U.S.-led United Nations Command in Korea, said in a recent interview before leaving for Washington, where he will be Army vice chief of staff. North Korea, however, "has the capabilities to attack the South with little or no warning. It is a nation geared for offensive military activity at a propitious time."

Wickham suggested that the timing could involve the outbreak of a crisis in the Middle East that would divert U.S. forces outside Korea, normally relied on to provide second-line support, to positions in the Persian Gulf.

It could involve a scramble for power in Pyongyang, where Kim, 70, reportedly has met with resistance to plans to install his son, Kim Chong Il, as his successor. The delicate balance on the peninsula could be upset, analysts suggest, by a prolonged political upheaval in the South on the order of the chaos triggered by the assassination of president Park Chung Hee in October 1979.

Tensions along the DMZ have risen since last August, when the North Koreans fired a missile that narrowly missed a U.S. SR71 spy plane flying off the southwestern coast of North Korea. In December, Pyongyang mounted what analysts here said was its largest military training maneuver to date in an area close to the DMZ. In April, four North Korean soldiers were killed on a northeastern sector of the truce line. Seoul said the four apparently were trying to defect and were killed on the North Korean side of the line, but Pyongyang said the casualties were suffered in a battle sparked by South Korean troops firing on a routine patrol.

Kim Chongwhi, director of national security research at the National Defense College in Seoul, said, should war come again, "it will be very short. It won't, like in 1950, last three years. The South wouldn't have time to bring its economic capabilities to bear and the U.S. wouldn't have time to bring forces and equipment from outside Korea." Raw military capabilities, he said, "will be very important."

To upgrade those capabilities, the United States has embarked on a buildup in South Korea that features replacing older equipment with F16 fighters, A10 tank-killing jets and sophisticated patrol aircraft and ships. There are also plans for transfer sales of about $2.5 billion worth of U.S. military hardware to Seoul in line with its new five-year force-improvement plan.

"We have a long way to go to assure that deterrence is achieved," Wickham said. Should U.S. forces be engaged in serious military hostilities elsewhere in the world, he said, "it would take longer than otherwise planned, but . . . we could still conduct a successful defense" of South Korea.

Once across the DMZ, some analysts estimate, it would take North Korean warplanes three minutes to swoop down on Seoul, and defensive positions along the border would be hard pressed to repel a sudden land attack coming in great force. Within two days, Pyongyang's Soviet-designed T62 tanks could mount an attack on the city while it might take up to a week for South Korean and U.S. forces to muster the reinforcements needed to repel an invasion.

Rejecting what he called "alarmist views," a knowledgeable foreign observer in Seoul said, "The North Korean threat is no more immediate now than in the '50s, '60s or '70s. The North attacked in 1950 because the U.S. pulled out its troops, and there was the perception that it wouldn't retaliate."