Douglas Feith, undersecretary of defence for policy, became on Wednesday the first senior Pentagon official to announce his resignation since the re-election of President George W. Bush.
Mr Feith told Donald Rumsfeld, defence secretary, that he would stay in the job until a replacement was found. His departure raises questions about whether the Pentagon is preparing to change key officials or policies.
As one of the most prominent neoconservative officials in the Bush administration, Mr Feith was one of the architects of the Iraq war and had responsibility for postwar reconstruction.
While Mr Rumsfeld praised Mr Feith, saying he had “earned the respect of civilian and military leaders across the government”, current and former administration officials said there was widespread relief at the news of his resignation.
A former senior administration official said the White House had been pushing for Mr Feith's departure for several months. He had developed a contentious relationship with other members of the administration, including the White House and the State Department, where one official said there was a “sigh of relief” at the news.
The official said Mr Feith's departure did not represent a change in policy, given that Mr Bush had reaffirmed many of the neoconservatives' views in his inaugural speech last week. He said that instead it was an effort to remove some officials who were lightning rods in the first administration.
Mr Feith drew criticism early in Mr Bush's first term by creating the Office of Strategic Influence, which he wanted to plant news stories with foreign media to influence policymakers. Mr Rumsfeld closed the office after details of its methods emerged in the press.
Mr Feith also had a difficult relationship with the State Department, battling with officials over pre-war intelligence estimates and reconstruction plans for Iraq. The former senior official said relations had deteriorated to the extent that some State Department officials refused to sit in the same room with Mr Feith.
Through the Office of Special Plans, which he created to gather and analyse intelligence independent of the Central Intelligence Agency, Mr Feith was instrumental in pushing intelligence reports that Saddam Hussein was linked to the terror attacks of September 11 2001. Mr Bush was later forced to admit that the administration had no evidence of any such links.
Mr Feith's relations with the uniformed military were equally strained. Retired General Tommy Franks, who commanded US forces during the invasion of Iraq, wrote disparagingly about Mr Feith in his autobiography American Soldier.
“I wasn't convinced that [Mr Rumsfeld] was always well served by his advice,” Gen Franks wrote. “Feith was a theorist whose ideas were often impractical…. He had a reputation for confusing abstract memoranda with results in the field.”
Scott Horton, president of the International League for Human Rights, said the Pentagon policy position was so important to the neoconservatives that they would push hard to make sure one of their members replaced Mr Feith.
But he said the White House would probably pick someone who attracted less criticism.
Mr Feith's office attracted attention last year after details were published of a Federal Bureau of Investigation inquiry into whether Lawrence Franklin, who worked for Mr Feith, passed classified information to the American Israel Public Affairs Committee and an Israeli diplomat. Some neoconservatives speculated yesterday that Scooter Libby, chief of staff to vice-president Dick Cheney and protégé of Paul Wolfowitz, deputy defence secretary, could replace Mr Feith at the Pentagon.
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