AS SECRETARY OF state, Colin L. Powell liked to think of himself as a latter-day George C. Marshall, a loyal soldier who did his best to carry out his president's policies even if he didn't entirely agree with them. On the most memorable day of his tenure, Feb. 5, 2003, he did just that, arguing the case against Saddam Hussein before the U.N. Security Council and relying on what turned out to be faulty intelligence to support a war about which he had serious reservations. But for much of the past four years, Mr. Powell was less a trouper than a dissident inside the Bush administration, a champion of moderate or traditional U.S. foreign policies and their architects in the State Department who sought to brake President Bush's sometimes radical shifts in direction.

For the most part, Mr. Powell lost his arguments with Vice President Cheney, Defense Secretary Donald H. Rumsfeld and other hard-liners -- on Iraq, on the Israeli-Palestinian conflict, on applying the Geneva Conventions to prisoners. He succeeded in persuading Mr. Bush to take the case of Iraq to the United Nations, and in moderating other initiatives. At times, however, infighting between State, Defense and the vice president's office had the effect of paralyzing the administration on crucial issues, such as Iran and North Korea. For that reason it's not surprising that Mr. Powell would have been willing, as The Post reports this morning, to stay in office only if given greater control over foreign policy -- or that Mr. Bush would not have urged him to stay on.

The possibility that Mr. Powell's influence, however weak, will no longer temper administration policies will dismay his many supporters, ranging from State's career diplomats to moderate Republicans in Congress and many foreign governments. They found the secretary of state to be one of the few sympathetic figures in an administration that tore up treaties, ignored Congress and sometimes scorned traditional U.S. allies. Yet Mr. Powell also must bear responsibility for the administration's many diplomatic failures. His reluctance to travel -- he spent less time abroad than any secretary of state in decades -- greatly reduced his ability to cajole foreign leaders and charm their publics at a time when effective American public diplomacy was desperately needed.

Notwithstanding his appearance at the United Nations, Mr. Powell could have done far more to build international support for the U.S. intervention in Iraq, employing his own prestige with countries such as Turkey or Mexico -- not to speak of Germany and France. He also could have argued more forcefully inside the administration for the postwar planning that was undertaken by his department but discarded by the White House. He soldiered on, defending the war publicly, but vivid accounts of his prewar doubts and cautions to President Bush were provided to influential journalists, such as The Post's Bob Woodward.

Mr. Powell's departure may well lead to fewer arguments and more consistent action by a second Bush administration as a team of officials closer to Mr. Cheney and Mr. Rumsfeld takes over at the State Department. Yet it is a measure of the stunning absence of accountability under Mr. Bush that it is Mr. Powell who leaves, while the architects of the failed and even disastrous policies he opposed, from postwar Iraq to Guantanamo Bay and Abu Ghraib, remain in office. Mr. Bush has signaled lately that he would like to repair some of the diplomatic damage of his first term, starting with U.S. relations with Europe. We trust he's sincere, but it's hard to be optimistic that he'll succeed without acknowledging that the secretary of state who lost all those first-term arguments, and who now has been let go, more often than not was right.