As it becomes more and more obvious that the South Korean Navy corvette Cheonan was sunk on March 26 by a North Korean torpedo, more and more voices are calling for cooler heads to prevail. Except, that is, for those who are calling for a strong, if not massive, military response to what, if confirmed, will be a clear act of aggression which violates the 1953 Armistice and thus invokes the U.S.-Republic of Korea security treaty.

I stress "if confirmed," since the ROK government has been very careful not to jump to any conclusion, as increasingly obvious as it appears to be becoming, without a thorough investigation of the wreckage. This is as it should be but still begs the question: What should South Korea and the U.S. do if a deliberate torpedo attack — clearly an act of war — is conclusively proven to be the cause?

I pride myself as being a lifelong member of the "cooler heads" club, but this does not mean doing nothing. Nor does it mean ruling out in advance — as many seem inclined to do — a number of different measured but appropriate military responses if North Korean culpability is proven.

Again, the first step is to demonstrate beyond a reasonable doubt that the sinking was the result of a deliberate hostile action. This will require maximum transparency in analyzing and discussing the evidence which is currently being obtained and examined. Given that 80 percent of South Korean citizens already believe Pyongyang was somehow responsible, it will be equally important — and perhaps more difficult — for the Lee Myung-bak government to convincingly demonstrate that such a conclusion cannot be reached.

If the evidence of North Korean hostile action is conclusive, then there must be a firm but measured response. The most appropriate vehicle for channeling this response is the United Nations Security Council. The South Korean government, in cooperation with the international investigation team (including U.S., Australian and Swedish experts), must make its case before the council. Pyongyang should then be provided an opportunity to explain its action and to identify and appropriately punish the guilty parties if it proves to be a rogue element. An official apology and reparations would also be in order. Such a response would and should result in the case being closed, beyond a clear warning that repeat offenses will be dealt with more harshly.

If Pyongyang refuses or, worse yet, if current rumors that the North Korean military officer in charge has actually been promoted as a reward for this egregious act prove to be true, then additional sanctions — the council's traditional weapon of choice — will not be enough.

Here is what ought to be done. The council should mandate, in addition to increased sanctions (not to mention stricter enforcement of those already on the books), that all North Korean submarines and torpedo boats are hereby restricted to port until further notice. Any that are determined to be under way should be deemed as legitimate targets for prosecution and destruction by the Seoul-based United Nations Command and the South Korea-U.S. Combined Forces Command. The CFC should further announce that it reserves the right to "render inoperable" any North Korean naval facility that bases the offending naval units. If the council refuses to take such firm action, then the CFC should unilaterally take this position.

Pyongyang is sure to brand such responses an act of war, as they have similarly declared any number of less forceful actions. But Pyongyang is not suicidal; it fully understands the risk of escalation and who would be the ultimate loser. It has been North Korea's belief that there will be no meaningful consequences, a belief unfortunately reinforced by history, that emboldens its actions. This mindset needs to be corrected.

Some will argue that such a response will undermine the prospects for a resumption of Six-Party Talks aimed at Korean peninsula denuclearization. Actually the reverse is true. It would be politically impossible for Seoul (and inappropriate and unwise for Washington) to return to such a dialogue — even if Pyongyang was willing, which it has thus far proven not to be — until the Cheonan matter is settled.

The ball is currently in Seoul's court. It must make the case and then take the lead in crafting an appropriate response. The U.S. must be seen as being in lockstep with its South Korean ally; Washington should not be seen as trying to hold back or water down any response endorsed by Seoul. To do so will call into question the U.S. commitment to South Korea and its other security allies.

Turning the other cheek or a gentle slap on the wrist is sure to result in continued North Korean acts of aggression. A firm but measured response along the lines suggested above seems the best way of ending the cycle of aggression and persuading Pyongyang that the international community is finally serious about putting an end to its unacceptable behavior.

Ralph Cossa is president of the Pacific Forum Center for Strategic and International Studies. He wrote this commentary for The Advertiser.