BY Paul Reynolds
World affairs correspondent, BBC News website
Colin Powell never quite found his place in a Bush administration dominated by neo-conservatives.
Powell and Bush: opposite directions
Nor did he make the transition from general to statesman.
His weakness was that he lacked the vision of the world held by his rivals. Colin Powell was no dove. He too believed in US power and influence but where others saw certainty, he saw complexity. This slowed him down and gave them the edge.
He seemed to find it more natural to follow an order than to give one.
And in the end, he lacked the ear of the president, without which a secretary of state is powerless.
The result was disengagement.
The confident figure who led the United States military in the war against Iraq in 1991 became something of a marginalised figure in the war against Iraq in 2003.
The one internal battle he won over Iraq - that the US should go to the UN - was soon overtaken by events as the United States went to war anyway.
He was all but humiliated when the aggressive briefing he gave in February 2003 to the Security Council about Iraq's alleged weapons of mass destruction turned out to be based on wrong intelligence.
It has also emerged that he was told of President George W Bush's decision to go to war after the Saudi ambassador to the United States.
In all this, he never acted disloyally. Indeed, President Bush relied on his military discipline and told him just before the Iraq war that it was time to put his uniform back on. He did so.
Liberal black Congressman Charles Rangel said of him: "Colin Powell is a military guy, and he doesn't care who he works for, he just salutes."
Colin Powell never quite found his place in a Bush administration dominated by neo-conservatives
Yet enough became known about his estrangement that his own deputy Richard Armitage indicated that he and his boss would be leaving if there was a second Bush term.
Most observers, though, were expecting a little more delay.
It makes his forthcoming trip to the Middle East a rather uncertain affair.
One of Colin Powell's weaknesses was his reluctance to engage in diplomacy first-hand. He became known in some quarters as the secretary of state for Washington.
He lacked the enthusiasm of a Henry Kissinger shuttling across the Middle East, though he did engage there regularly and in person.
And he was not without his successes. He argued strongly for a Palestinian state and it was in no small way due to him that Mr Bush became the first US president to support this policy.
His lack of diplomatic dynamism left him with no counterbalance when the neo-conservatives led by Vice-President Cheney, Defence Secretary Rumsfeld and Deputy Defence Secretary Paul Wolfowitz weighed in.
It is doubtful whether the neo-conservatives had their own disengagement plan for Colin Powell. He was a very useful presenter of US policy, given that he has been the first African-American secretary of state.
But they will probably not be too sad to see him go.