Though critical and long-awaited, the reactor shutdown may also be the easiest achievement. It essentially restores the status quo that existed in 2002 — except that now North Korea is believed to have enough plutonium fuel for eight or more weapons, in addition to the one or two it is believed to have manufactured when Mr. Bush’s father was in office.

The challenge now, which experts believe will be far more difficult, is to convince North Korea to reveal and disgorge its arsenal. Almost all of that was produced starting in 2003, while the United States was distracted by the invasion of Iraq and its aftermath.

The February accord commits North Korea to eventually ridding itself of that fuel or the weapons it may have been turned into. But it sets no deadlines, and getting the North to take those steps will require a second negotiation.

“I could imagine that the next steps could extend beyond this administration,” William J. Perry, a former defense secretary under President Clinton, said in an interview in his office at Stanford University on Friday. “And the North Koreans will demand a pretty high price for that.”

North Korea meets its negotiating partners again in Beijing on July 18, and in a statement yesterday the State Department said that it planned to use the session “to make rapid progress in implementing the next phase set forth in the Feb. 13 agreement.”

The initial North Korean steps may also give some additional leverage in Washington to Mr. Hill and Secretary of State Condoleezza Rice as they try to reverse some of positions that the administration took in the first term, when some, including Vice President Dick Cheney, refused to negotiate with the country and looked for ways to speed the demise of Mr. Kim’s government.

Mr. Hill and Ms. Rice quietly dropped the American insistence that North Korea would not be rewarded for reversing the steps it took in 2003, when it expelled the inspectors, increased the production of bomb material and withdrew from the Nuclear Nonproliferation Treaty.