After the sinking of a South Korean warship by a North Korean torpedo, the U.S. is planning to help the South strengthen its navy. The country is still vulnerable to its unpredictable neighbor despite years of training and massive spending—the U.S. has long worked with the South on plans to prevent an invasion from the North and would be sucked into any war there. Yet only the South Korean navy is guarding the waters where the warship was sunk, a security hole that the countries intend to quickly fix. The bankrupt North can no longer invest in troop buildup or conventional weapons, so it has invested in stealth technology that can cause the South heavy damage but would not win a sustained war. In the near term, the U.S. and South Korea will conduct joint training exercises in the next few weeks, but the long-term focus will shift to the technically difficult defense against miniature submarines.