President Reagan's visit to South Korea next month has now become of paramount importance in reassuring and restraining this tense frustrated nation. In the aftermath of an Oct. 9 explosion in Rangoon, Burma - which South Korea blames on the communist North - that killed 17 visiting Koreans, including five of the most important officials in President Chun Doo Hwan's government, Mr. Reagan has to reinforce American policy that, no matter what the provocation short of invasion, Seoul must continue to turn the other cheek.
This is the price South Korea has to pay in lost pride in order to be assured of full American support should North Korean repeat its 1950 military sortie into the south.
And there are signs that for the South this lack of ability to strike back is building up a dangerous head of steam in some sectors.
The Seoul government is perfectly satisfied from all the evidence that the bomb explosion was triggered with aremote control devise by a North Korean suicide squad, two members of which allegedly have been killed and a third captured so far by Burmese security authorities.
The problem is to get the rest of the world to accept this claim and to act on it - condemning and applying sanctions against the North - in a way that will assuage frustrated public opinion and head off any build up of ''march north'' pressures on the Chun government.
South Koreans note, for example, just how weak, short-lived or nonexistant were the actions taken by the rest of the world against the Soviet Union for shooting down a Korean airliner Sept. 1, when it strayed into Soviet airspace, killing 269 people. So they don't feel too encouraged this time.
Commented one Western diplomat: ''The Koreans have gone through a bad few weeks in which they feel they have been shown as a small country that can be pushed around with impunity. . . . They don't like it, and the amazing thing is just how much they have been prepared to swallow.
''The obvious question, though, is: just how much more can they take without bursting and striking back?''
Referring to the airliner incident, one Western source observed: ''There was no target against which the public could vent its anger. There was no Russian Embassy they could throw stones at, and burning a few Russian flags or effigies is not very satisfying emotionally.
''The same thing has happened over the Rangoon bombing. There isn't any tangible target for them to let off steam.''
According to an American Embassy official: ''There is one important difference, however. They couldn't do much against the Russians, but it is a different case with North Korea. . . . That's a level more in keeping with their longstanding major preoccupation and their strength.''
Led by some of the media, there have been loud demands for revenge and a feeling of ''let's get at them (the North).''
Calling in his top commanders to discuss ways to establish a stronger defense posture against any northern threat, Defense Minister Yoon Sung Min said the North was on a semi-war footing and would ''have to bear all responsibilites for the worse situation that might stem from such provocations (as the Rangoon bombing). There are limits to our patience.''
Also meeting his top military commanders, President Chun - who only escaped the bombing by being behind his official schedule - described the incident as a virtual declaration of war by North Korea.
Had he been killed, Chun claimed, northern troops would immediately have launched a major attack across the demilitarized zone.
The President said the only answer was to build up the South's ability to overpower the North so that the ''warmongers'' in Pyongyang would ''cave in of themselves.''
Yet meeting visiting Japanese Foreign Minister Shintaro Abe last Thursday, Chun was quoted as giving a reassurance he would not retaliate militarily but would deal with the Rangoon bombing diplomatically.
But there were unconfirmed Japanese reports that Chun also told Abe he was ''suppressing disquieting moves among the armed forces, particularly among young officers,'' in connection with ways to react to the bomb incident.
Well-informed sources say there are voices demanding action to satisfy national pride, using the argument that ''if Israel could do it, why not us?'' (Apparently a reference to Israel's invasion of Lebanon to end the threat from PLO guerrillas).
But there are also more moderate voices who argue along the lines that ''our entire country was devastated by war 30 years or so ago, but we have recovered and built an economically strong nation that is starting to gain important recognition in the world. Why risk all that now by starting another war? It's far better to go on swallowing our pride and winning international sympathy by our tolerance of every northern atrocity.''
Speaking of the government's dilemma, Assistant Information Minister Choi Tae Soon commented: ''We are trying very hard to appease public indignation and frustration. We cannot simply act according to sentiment, but unless we ourselves are able to do something concrete, the frustration could turn against the government. . . . The people might then do wild things domestically.''
He said the government's main hope was that the evidence of northern involvement would be so strong that, internationally, bodies and individual countries would institute various sancitions to condemn Pyongyang.
But as he and Western diplomats noted, there is an equal frustration here. As one source noted: ''North Korea is already isolated diplomatically and has little trade with the non-communist bloc. There is not much leverage in the way of sanctions that can possibly be applied against it.''
Some obervers say that if the bombing happened two years ago, things might have been different. They say that this year Chun has shown greater confidence in leading the country and is considered less likely to react to his narrow escape as did his predecessor, the late Park Chung Hee.
After a North Korean-trained assassin's bullet missed him and instead killed his wife in 1974, Park reacted by developing a ''fortess mentality,'' becoming more authoritarian and intolerant of dissent.
All the evidence so far, including the choice of liberals in his new cabinet, suggests Chun for the moment at least does not feel the need to tighten up domestically to better face the continued northern threat.
The main US concern in the past few days has been to encourage southern restraint by asserting in strongest terms Washington's commitment to guarantee South Korean security.
US officials said this was one reason why Defense Secretary Caspar Weinberger was sent to Seoul for the national funeral for the bombing victims. The 40,000 American troops here have been on heightened alert along with their 600,000 South Korean counterparts, while the aircraft carrier Carl Vinson has remained off the Korean coast as a symbolic gesture.
Gen. Robert Sennewald, commander in chief of the South Korean-US combined forces, called off a scheduled visit to Washington this week, explaining: ''During this important period following the tragic bomb blast . . . my place is here where my fundamental responsibilities reside . . . doing everything (to show) the US stands resolutely with the Republic of Korea in pursuit of peace and security.''
This is the message that President Reagan is expected to carry when he comes here, as there is every sign the Koreans are looking to him to bolster their frustated feelings of inadequacy.
cl11.5 Tensions are already high along the demilitarized zone marked by the 38th parallel, however, and the Reagan visit is likely to intensify rather than ease these. In recent days, North Korea has been broadcasting repeated tirades against the visit, including such threats as ''he (Reagan) will never leave (Seoul) alive'' and that the US Embassy will be turned into a ''pile of rubble.''
American and South Korean security officials don't discount the seriousness of the threats, but are confident they can cope. A foreign ministry spokesman said that under the circumstances, ''The visit by President Reagan has become even more important than ever before for the stability of (South) Korea.''