U.S. WILL AID NAVAL DEFENSE OF SOUTH KOREA (EMBARGO: No electronic distribution, Web posting or street sales before 12:01 a.m. ET, Monday. No exceptions for any reasons. EMBARGO set by source.) (This article is part of TIMES EXPRESS. It is a condensed version of a story that will appear in tomorrow's New York Times.) Choe Sang-hun contributed reporting from Seoul, South Korea. 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.' 