Seoul — Should a second Korean war ever break out, the American who would meet the challenge right now has been through it all before. Robert W. Sennewald, a 1951 graduate of the Reserve Officers Training Corps, saw extensive combat duty in Korea as a young artillery officer before the fighting ended in a truce in 1953.
Now, after a 30-year career that has also seen combat duty in Vietnam, command of artillery batteries in Europe and the United States, and a wide variety of other top commands, training, and intelligence assignments in the United States, General Sennewald is back in Korea in the supreme military role begun by Gen. Douglas MacArthur. This weekend he will give President Reagan a personal tour of Korea's demilitarized zone (DMZ).
As befits America's complex and important role on the Korean peninsula, Sennewald wears several different hats.
He heads the so-called United Nations Command, maintaining the principle that , as it was during the fighting phase, maintenance of South Korea's security is a UN and not just a US responsibility.
The UN aspect these days, however, really comes down to administering the continued uneasy truce between North and South, with other UN member countries usually becoming involved only when investigating complaints of truce violations - exchanges of shots or infiltration of the heavily fortified DMZ at the 38th parallel.
General Sennewald also commands US forces in Korea, which now number about 39 ,000. He would like more, he says, but, since he has to make do with that number , he is intent on making steady improvements in both their fighting ability and the quality of the equipment they have to use to repel any invasion.
Finally, the native of St. Louis, Missouri, also has a highly sensitive position as head of the combined South Korea-United States Forces Command.
As one official described it: ''Of course, the Koreans have their own commanders. But when it comes to an invasion, with the Koreans and Americans fighting side by side, it is General Sennewald who will make the ultimate decisions on where and how to fight.''
The general has to act in many ways as a diplomat, integrating the two forces without disturbing national pride, giving the host country the strong assurances it needs of American military support, while at the same time acting as a restraining influence against any precipitate movement in the South Korean military to ''march north.''
Just such a sentiment reportedly has existed among certain elements since the Oct. 9 bomb explosion in Rangoon, Burma, that killed five South Korean Cabinet ministers and narrowly missed President Chun Doo Hwan. An investigation by the Burmese government has laid blame for the attack squarely on North Korea.
The delicate role of the US commander in South Korea, however, raises a serious question as to how far it draws the US into domestic Korean politics to support the particular regime in power.
It was an issue faced by General Sennewald's predecessor, Gen. John A. Wickham Jr., in 1980 when an antigovernment uprising to protest lack of political freedom broke out in and around the city of Kwangju.
The then military-based government in Seoul eventually crushed the unrest. Korean Army units were released from their duties by the US commander and were rushed to the scene from their normal station on the DMZ, where American forces temporarily filled the vacuum. The Kwangju incident resulted in the loss of an estimated 2,000 Korean lives.
To some Koreans, the US was just as much a party to the repression as if American troops had been sent to Kwangju as well. And some thoughtful Americans felt the US should have tried to use its influence through the joint command to persuade the Seoul government not to be so heavy-handed against dissent.
But it remains far from clear how much influence the US really has over Korean military actions, especially when these fall outside the clearly defined area of coordinating reaction to a North Korean invasion.
In his occasional public utterances, General Sennewald has stuck strictly to the narrow definition of his role as joint commander: developing the capability of his twin force to cope with attack.
The northern threat is as real today as it was in 1950, the general says, and there have been dramatic improvements in North Korea's fighting capability in the past ten years. Some 600,000 South Korean and 39,000 American troops face across the demilitarized zone a highly trained and motivated force of at least 750,000 men.
The combined southern forces are outgunned on a weapon-to-weapon count, but ''we have qualitative superiority that helps balance the equation,'' Sennewald said in a report which marked the 30th anniversary of the 1953 armistice agreement on July 27.
In tanks, for example, the North has twice as many as the South. The general noted the North had introduced hundreds of locally built copies of the Soviet T- 62 tank.
Northern forces have Soviet FROG ground-to-ground missiles capable of striking beyond the southern capital, Seoul, which is only a 45-minute drive from the DMZ, a distance that puts it squarely in the front line.
North Korea's warplanes outnumber the South 2 to 1, but the US makes up for much of this with quality in its F-16s and A-10 close support bombers. Pyongyang , says General Sennewald, also maintains the world's largest unconventional warfare - some 30 brigades of commando and other special operation elements.
In a recent speech, Sennewald admitted he was concerned about the infiltration capability of the North.
''I believe they are a formidable foe, well trained, well equipped and are something we should, and are, concerned about.'' Yet the general is convinced that on his side ''we have the capability to deter North Korean aggression or to defeat it on the battlefield. . . . The North Koreans will pay the price if they attack.''
In a rare press appearance last June, the general gave an outline of the strategy he would adopt to meet a northern invasion. ''Our planning is based on a combined forward defense. . . . We envisage a series of battles simultaneously. The North Koreans have first, second, and third echelons (lines of defense), and we hope to place at risk all North Korean forces at once.''
Summing up, he said: ''The (South's) combined forces are equipped, trained, and conditioned for war - under any condition and at any place North Korea may elect.''