NATO Secretary General George Robertson says he will step down in December, despite pressure to stay in his job for another year.
Mr. Robertson says several governments asked him to continue for an optional fifth year, but he believes four years is the right term in what he called "a demanding and onerous job."
Observers say the resignation is a surprise.
Mr. Robertson received praise for a number of achievements. He is the only NATO chief to have invoked the alliance's mutual defense clause, a move he took after the September 11 attacks on the United States.
He also negotiated last year's Prague summit agreement to admit seven more countries from central and eastern Europe and to modernize NATO's aging armed forces. He maintained NATO's peacekeeping missions in Bosnia and Kosovo, and helped negotiate an end to a crisis in Macedonia, a dispute that regional experts say could have sparked a new Balkan war.
Despite these successes, officials say in recent months there have been growing tensions within the alliance over Iraq. The United States and Britain have been stepping up their preparations for a possible military action, while France, Germany, Turkey and other European allies are opposed.
A NATO official says there is no obvious successor to Mr. Robertson. Diplomats say there is speculation that Polish President Aleksander Kwasniewski could be a candidate. But his presidential term does not expire until the end of 2005.