NEAR THE DMZ, South Korea - All the public reassurances from Washington cannot dispel the foreboding among U.S. and South Korean generals that President Carter's planned withdrawal of U.S. infantrymen from Korea will bring communist legions down across this most heavily fortified border in the world.

If there are not American foot soldiers as a trip wire on the demilitarized zone separating North and South Korea, will the president unhesitatingly dispatch U.S. bombers? Without them, superior communist firepower will surely sweep through South Korean defenses into the Sprawling capital of Seoul.

Doubts here about the U.S. commitment must be shared by Kim II Sung. North Korea's dictator, who at 66 sees time running out on his dream of unifying the Korean peninsula by force. His last opportunity may be 1982, when Carter plans to withdraw the last battalions of the U.S. 2nd Infantry Division. That makes the mood among the military here even bleaker today than on our last visit in 1975, just after the fall of Saigon.

Kim II Sung's intentions seem less cloudy when viewed from the DMZ than from Washington. The continuing military buildup has widened the communist lead in tanks, artillery and overall firepower. This, added to North Korea's extensive defense industry, enables Kim to invade without permission of either Moscow or Peking.

His plan: capture Seoul even more quickly than the three days it took in the 1950 invasion, then seek negotiations to unify the peninsula on communist terms and radically transform Northeast Asia's balance of power. It's a no-lose proposition. Should his invasion fail, Kim's communist big brothers would not let him lose his country now any more than in 1951 when Chinese communist troops intervened.

But he will not move if the odds are not favorable. The obstacle to taking Seoul is the forward defense strategy instituted in 1974 by now retired Lt. Gen. James Hollingsworth, then commanding the western front. The DMZ has been transformed since our visit three years ago.

Flying by helicopter over the invazion routes, we observed hills honey-combed with concrete bunders. We visited Hill 229, one of two mountains turned into little Maginot Lines by the South Koreans, with underground tunnels and scores of bunkers. A 20-foot hight, 8-foot wide concrete wall, crossing the peninsula, was begun six months ago (against U.S. military advice).

Nevertheless, the vital ingredient in the Hollingsworth strategy is massive U.S. air support, particularly B52 bombers, to neutralize superior communist firepower.

Generals here would like 48 to 72 hours warning of invasion, but U.S. intelligence considers 18 to 24 hours more realistic. That permits no dilly-dallying at the White House, considering the time needed to switch bomb racks of the B52s based on Guam from nuclear to conventional.

One key adviser to South Korean President Park Chung Hee suspects that President Carter would wait to see how the battle goes before deciding whether to send bombers. But a secret report recently quoted two South Korean division commanders as fearing how their troops would behave if U.S. bombers did not appear overhead promptly after the invasion. Thus, an American president waiting for the outcome of the fight before acting would profoundly affect its outcome by waiting.

There would be no waiting if Kim II Sung attacked today. Holding a tiny sector on the western front is a lone American battalion from the 9th "Manchu" Regiment (named for service in the Boxer Rebellion). Although it is supposed to quickly retire to the rear, the U.S. troops would suffer casualties. They truly are a trip wire.

Highly publicized substitutes for the 2nd Infantry Division are scoffed at by South Korean generals. A long-scheduled addition of 12 F4 jet fighters was billed as a new U.S. squadron (through a squadron's strength is 18 to 24 jets) arriving with ZDefense Secretary Harold Brown Nov. 8. That may have reassured Koreans in the street but not the ministry of national defense. "It is ridiculous - exchanging 12 jets for an infantry division," one senior Korean general told us.

Relatively few border incidents in the last 18 months reflects Kim's waiting for the last of the 2nd Division to go. But there are increased communnist troop concentrations on the DMZ and obvious preparations for invasion. We walked through the third underground invasion tunnel discovered so far, and U.S. Army experts believe there are at least 10 more tunnels.

To avert a catastrophe, South Korea's preoccupation today is to postpone the 1982 troop pullout. This transcends Koreagate, human rights and trade problems as the focus of U.S. Korean relations.