SEOUL, South Korea — The United States and South Korea face an imminent problem and they do not seem to have the answer. At what point do rhetoric and war games escalate into serious reprisals?

When it comes to rhetoric, there’s plenty of it.

South Korean President Lee Myung Bak has promised, more than once, “stern retaliation” the next time the North attacks. Kim Kwan Jin, the new defense minister, said the South will “retaliate immediately and strongly until they completely surrender” and ordered war games this week off South Korea’s coasts — though not around the same island in the Yellow Sea that North Koreans hit with an artillery barrage two weeks ago.

North Korea has said tensions are worsening “by the hour,” and President Lee vowed Tuesday to turn the South’s five Yellow Sea islands within eyesight of the North Korean coastline into “military fortresses,” defended with many more troops and powerful weapons. Who would dispute the need to take these overdue measures?

The hard part, though, is knowing when to raise the stakes — when, for instance, to send South Korean fighter planes against targets in the North.

As of now, South Korean planes are only armed for air-to-air combat against North Korean planes intruding into South Korean air space — something that's only happened in three cases when pilots flew their MiGs south to defect. Kim Kwan Jin said South Korean aircraft will be ordered to attack the sources of fire the next time the North stages a shock attack, but it’s far from clear when to carry out such a threat.

What, exactly, is the way to respond if the North Koreans fire a few shells and hit no one? Or if they inflict casualties far from population centers? For that matter, what if they stage a gunfight in a remote region along the 155-mile-long demilitarized zone that’s divided the two Koreas since the Korean War?

If that’s “the forgotten war” then the intrusions afterwards, in which dozens of Americans and South Koreans were killed, are hardly remembered at all. Would renewal of those exchanges of fire in lightly populated, hilly regions trigger more than perfunctory counter-fire?

The sense, from listening to American and South Korean military people, is that no one knows what to do. The Americans discuss options, planning and the weapons they believe would stop North Korea dead in its tracks.

It’s unlikely, however, the North would be so foolish as to go to “all-out war,” a phrase often heard from Pyongyang. The real danger now are isolated quick hits — episodes that make headlines but are over quickly and don’t interfere with the lives of the 50 million South Koreans to whom the shooting remains a distant apparition.

That’s where the politicians and military people do not have the answers. They can’t decide whether to send planes over the North, to keep on firing until the source of the attack is destroyed, to fire at supply lines and troop concentrations or, if none is apparent, to look for targets elsewhere.

One reason for the uncertainty of the South Koreans is they never can be sure what the Americans are thinking. True, U.S. President Barack Obama has seemed firm in his pronouncements. Yes, he ordered the aircraft carrier George Washington, a 97,000-ton nuclear-powered behemoth, to lead a U.S. strike force into war games with South Korean ships in the Yellow Sea after the North's Nov. 23 attack. That was a significant move, symbolically and practically, since the U.S. had canceled a plan to send in the George Washington after the sinking of the South Korean navy corvette the Cheonan on March 26, for fear of upsetting the Chinese.

Symbolically, the presence of the George Washington showed the Americans shoulder-to-shoulder with the South Koreans — willing to deploy their biggest force in defiance of China’s claims to the Yellow Sea as an extension of Chinese territory. Practically, the Americans and South Koreans gained experience in coordinating complicated missions. If war were to break out, those lessons will prove indispensable. The Americans have followed up on that display, moreover, with still bigger exercises with Japanese forces, again to the consternation of the Chinese.

Still, the overriding question remains: How often does North Korea have to prick the hide of South Korea before the South acts forcefully to deter the North from striking again?

The initial response to asymmetric warfare need not be to send the George Washington to the rescue. Before that happens, South Korean artillery and fighter aircraft have to pound supply points, gun positions and harbors identified as targets in the North. If a North Korean vessel fires on a South Korean target, the answer may be not only to sink the attacker but also to bomb the naval base that harbored the vessel that mounted the attack.

Nobody ever considered that response in the wake of the sinking of the Cheonan. Bureaucrats and politicians cringe at the risks. Might North Korea then turn its artillery on the Seoul-Incheon megalopolis of more than 20 million? Would North Koreans fire at lesser population centers closer to the demilitarized zone — or attack a South Korean guard post south of the line? How should the South Koreans respond then, and when would their American ally join the fray?

The answer is the Americans and South Koreans have to set a “red line” beyond which they will not let the North Koreans get away in sneering triumph after more surprises. They need to challenge North Korea, to be ready to fight for the integrity of South Korea and the region.

The existence of the North’s ultimate club, its nuclear stockpile, should hardly keep U.S. and South Korean forces from fighting back after “incidents” that will go on and on, in more terrible form, unless answered with more than rhetoric — or even war games.

Donald Kirk is the author, most recently, of "Korea Betrayed: Kim Dae Jung and Sunshine."