Fear of war has become a new constant for the Korean peninsula. On Monday South Korea initiated a military exercise in the Yellow Sea and North Korea threatened to retaliate. Seoul went ahead without any response from the North, but the region retains the feel of a bomb with an unstable fuse.

In the short term Washington has no choice but to uphold its alliance obligations to the South. However, Pyongyang’s increasingly erratic behavior offers a dramatic reminder of the most important cost of the unilateral security guarantee: the threat of war.

The alliance was created at a different time in a different world—1953, after the conclusion of a war which had devastated the peninsula. Only U.S. military support preserved South Korea’s independence. Since then the South has developed economically and is well able to protect itself. The U.S. should begin turning over defense responsibilities to Seoul, with an expeditious withdrawal of all American troops. The defense treaty, with America’s promise to forever guard the South, irrespective of circumstance, should be turned into a framework for future cooperation in cases of mutual interest.

The U.S. no longer can afford to maintain Cold War alliances as if the Cold War still existed. Commitments like that to South Korea are expensive, since they drive America’s military budget. More important, as we see in Northeast Asia, alliances also increase the possibility of war for the U.S. It is time to update America’s military commitments to reflect today’s world.