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?''
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