WASHINGTON — Surprised by how easily a South Korean warship was sunk by what an international investigation concluded was a North Korean torpedo fired from a midget submarine, senior American officials say they are planning a long-term program to plug major gaps in the South’s naval defenses.

They said the sinking revealed that years of spending and training had still left the country vulnerable to surprise attack.

The discovery of the weaknesses in South Korea caught officials in both countries off guard. As South Korea has rocketed into the ranks of the world’s top economies, it has invested billions of dollars to bolster its defenses and to help refine one of the oldest war plans in the Pentagon’s library: a joint strategy with the United States to repel and defeat a North Korean invasion.

But the shallow waters where the attack occurred are patrolled only by South Korea’s navy, and South Korean officials confirmed in interviews that the sinking of the warship, the Cheonan, which killed 46 sailors, revealed a gap that the American military must help address.

The United States — pledged to defend its ally but stretched thin by the wars in Afghanistan and Iraq — would be drawn into any conflict. But it has been able to reduce its forces on the Korean Peninsula by relying on South Korea’s increased military spending. Senior Pentagon officials stress that firepower sent to the region by warplanes and warships would more than compensate for the drop in American troop levels there in the event of war.

But the attack was evidence, the officials say, of how North Korea has compensated for the fact that it is so bankrupt that it can no longer train its troops or buy the technology needed to fight a conventional war. So it has instead invested heavily in stealthy, hard-to-detect technologies that can inflict significant damage, even if it could not win a sustained conflict.

Building a small arsenal of nuclear weapons is another big element of the Northern strategy — a double-faceted deterrent allowing it to threaten a nuclear attack or to sell the technology or weapons in order to head off retaliation even for an act of war like sinking South Korean ships.

In an interview last week, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said that the joint training exercise with South Korea planned just off the country’s coast in the next few weeks represented only the “near-term piece” of a larger strategy to prevent a recurrence of the kind of shock the South experienced as it watched one of its ships sunk without warning. But the longer-range effort will be finding ways to detect, track and counter the miniature submarines, which he called “a very difficult technical, tactical problem.”

“Longer term, it is a skill set that we are going to continue to press on,” Admiral Mullen said. “Clearly, we don’t want that to happen again. We don’t want to give that option to North Korea in the future. Period. We want to take it away.”

American and South Korean officials declined to describe details of the coming joint exercises, except to say that they would focus on practicing antisubmarine warfare techniques and the interdiction of cargo vessels carrying prohibited nuclear materials and banned weapons.

To counter the unexpected ability of midget submarines to take on full warships, the long-term fix will mean greatly expanding South Korea’s antisubmarine network to cover vast stretches of water previously thought to be too shallow to warrant monitoring closely — with sonar and air patrols, for instance. That would include costly investment in new technologies, as well as significant time spent determining new techniques for the South Korean military.

North Korea presents an adversary with a complicated mix of strengths and weaknesses, said senior American officers.

According to a recent strategic assessment by the American military based on the Korean Peninsula, the North has spent its dwindling treasury to build an arsenal able to start armed provocations “with little or no warning.” These attacks would be specifically designed for “affecting economic and political stability in the region” — exactly what happened in the attack on the Cheonan, which the South Korean military and experts from five other countries determined was carried out by a North Korean midget submarine firing a powerful torpedo.

Admiral Mullen and other officials said they believed the Cheonan episode might be just the first of several to come. “North Korea is predictable in one sense: that it is unpredictable in what it is going to do,” he said. “North Korea goes through these cycles. I worry a great deal that this isn’t the last thing we are going to see.”

High-ranking South Korean officials acknowledge that the sinking was a shock.

“As the Americans didn’t anticipate 9/11, we were not prepared for this attack,” one South Korean military official said. “While we were preoccupied with arming our military with high-tech weapons, we have not prepared ourselves against asymmetrical-weapons attack by the North.”

The South Korean military was well aware that the North had submarines — around 70, according to current estimates. But the focus had been on North Korea’s using larger conventional submarines to infiltrate agents or commandos into the South, as it had in the past, not on midget submarines sophisticated enough to sink a major surface warship.

“We believe that this is the beginning of North Korea’s asymmetrical military provocations employing conventional weapons,” said the South Korean official, who spoke on the condition of anonymity to describe the military’s internal analysis. “They will use such provocations to ratchet up pressure on the U.S. and South Korea. The Cheonan sinking is an underwater terrorist attack, and this is the beginning of such attacks.”

Though it is considered unlikely, the threat of a conventional war with North Korea is still an issue, too, officials said.

The American military’s most recent “strategic digest” assessing both the strengths of the United States-South Korea alliance and the continuing threat from the North notes that North Korea’s military is “outfitted with aging and unsophisticated equipment.”

Even so, 70 percent of North Korea’s ground forces — part of the fourth-largest armed force in the world — remain staged within about 60 miles of the demilitarized zone with the South. In that arsenal are 250 long-range artillery systems able to strike the Seoul metropolitan area.

“While qualitatively inferior, resource-constrained and incapable of sustained maneuver, North Korea’s military forces retain the capability to inflict lethal, catastrophic destruction,” said the assessment, approved by Gen. Walter L. Sharp, commander of American and United Nations forces in South Korea.

There are about 28,500 American forces in South Korea today, significantly fewer than before the wars in Afghanistan and Iraq. The South Korean military has maintained its armed forces at a consistent number between 600,000 and 700,000, and has steadily modernized based on its economic dynamism.

The North has an active-duty military estimated at 1.2 million, with between five million and seven million in the reserves.

But many are poorly trained, or put to work building housing or seeking out opponents of Kim Jong-il’s government. The best trained, best equipped and best paid of them are North Korea’s special operations forces, numbering about 80,000 and described by the American military as “tough, well-trained and profoundly loyal.” Their mission is to infiltrate the South for intelligence gathering and for “asymmetric attacks against a range of critical civilian infrastructure and military targets.”