Jimmy Carter's foreign policy struck its first snag in South Korea and never fully recovered. So it is fit that the Reagan administration, despite a little overeffusiveness, welcome early and with a show of harmony President Chun Doo Hwan.

For Korea is an American omen. The right relation with Seoul offers a handle on security throughout Asia. It can also mark a sensible approach to the awkward problem of balancing between security interests and human rights.

The American stake in Asia now rivals, and perhaps even surpasses, the stake in Europe. Mainland China has become a foremost strategic ally against Russia. Japan is an exonomic giant with global strength. South Korea, Twaiwan, Hong Kong and Singapore join Japan as models of surging economic growth. The Philippines, Thailand, Indonesia and Australia are important trading partners for this country.

All those nations look to the United States for security against Russia and its two Asian allies -- Vietnam and North Korea. The chief American base in the area is South Korea. The chief American base in the area is South Korea. It is there that this country maintains 40,000 ground troops, a tactical air force and tactical nuclear weapons. It is over South Korea, first of all, that Washington holds the umbrella of the nuclear deterrent.

So when Jimmy Carter, as a candidate, declared that he planned to pull all American troops out of South Korea, a tremor ran through Asia. It shook Japan and China, and the Philippines and Thailand and Indonesia as well as South Korea.

The worried Asians mustered allies in the American military, in Congress and within the Carter administration. Eventually, thanks chiefly to Secretary of Defense Harold Brown, the president was turned around on troop withdrawal. But not before grave doubts had been raised among this country's allies in Asia.

The Reagan administration approaches the problem of Asian security from the other side. The cordial reception of President Chun is a mark of solidarity. There is no talk of pulling out U.S. troops. Rather, the emphasis is on enhancement of South Korea's defense capabilities. The signal is thus flashed from Tokyo through Peking to Bangkok and Jakarta that the United States is standing firm in Asia.

To be sure, two American brigades do not have to stand nose-to-nose with North Korean troops along the demilitarized zone forever. Nor does Washington have to give to Seoul weapons so sophisticated that the Russians will feel constrained to upgrade the military equipment they make available to the North Korean dictator, Kim Ii Sung.

On the contrary, shortages in American military personnel and the special demands of the Persian Gulf argue for an eventual drawdown of some U.S. forces in South Korea. The growing economic difficulties of North Korea suggest an eventual stabilization of the peninsula and a lid on the weapons given to each country.

But withdrawals and limitations ought to be discussed privately between friendly governments, not publicly by eager candidates. They should be taken up not in the political context, but within the context of a strategic view that includes Asia and the rest of the world.

Similarly with respect to human rights. South Korea is certainly no democracy. Rule is arbitrary, even illegitimate, and the military and police behave in a very rough manner.

But the public criticism of the Carter days availed nothing. On the contrary, when President Park Chung Hee was assassinated, he was replaced by the far more repressive regime of President Chun. The Carter administration had poor contacts with Chun, and the Korean leader went out of his way to show contempt for Washington's preoccupation with human rights.

The Reagan administration, by reassuring Chun of the security issue, has managed some human rights gains. It has achieved commutation of the death sentence imposed on the Korean opposition leader, Kim Dae Jung. It has also achieved an end to martial law. There is at least a chance that quiet until it eventually reaches the degree of openness appropriate to the booming Korean economy.

None of this argues that the essence of good policy is to do the opposite of whatever Carter did. There was a touch of such perversity in the administration's enthusiasm for Chun, and President Reagan and Secretary Haig may later regret they had not been more discriminating with their favor. Especially as they seek to develop new approaches in dealing with the European allies, and in the Middle East and Latin America. Still, in Asia the lines are drawn firmly, and this country has cause to stick to its gains and its guns.