Scrambling to put more boots on the ground in Iraq, the US said this past weekend that it plans to transfer about 10 percent of its 37,000 troops in South Korea.
In mild shock, Seoul's security chiefs are meeting Tuesday to see if they can make up for the gap in military defenses against North Korea.
The transfer would be the first such major redeployment of forces since the Korean War armistice in 1953, and perhaps the start of a permanent US drawdown on the divided peninsula. But it also sends this message to Seoul's new left-leaning government: The world's 12th-largest economy can do more to beef up its own military, especially when the US needs help.
South Korea has relied on the US troop presence for stability to help its economy grow. But its Army has slipped in quality over the years while Seoul gradually has come to see North Korea as less of a threat. In contrast, the US sees a greater threat to itself from North Korea's possible capability to launch nuclear-tipped missiles.
Such a divergence of views and interests requires careful management. For starters, President Roh Moo-hyun and his ruling Uri Party can send the 3,000 troops to Iraq that South Korea promised last year.
Once a harsh US critic, Mr. Roh is eager to redefine ties with Washington and get Seoul out from under the US shadow. He has plans to reform the military, but now, with the US troop transfer, he'll need to give those reforms as high a priority as his more-touted economic reforms.
US troops are in the south to slow down an invasion, not defeat North Korea. They're a tripwire to ensure American commitment. Seoul can rely on that support if it does more to improve its professional Army.