In this security-conscious era, a visit to Number 10 is a shock to a White House alumnus. You stroll right up to a guardhouse barely 50 yards away from the famous black door, show a photograph of yourself, go through a metal detector, walk to the house and simply press the doorbell. Then, you are shown to a chair in the same lobby through which the Prime Minister himself must pass on his way in and out of the building.

Even the Governor of the State of New York is surrounded with more pomp than the elected leader of the world’s fourth-largest economy.

Office space isn’t everything, of course. An American president has grander premises, bigger motorcades and a snazzier plane than a prime minister. But compare the two leaders’ legal powers, and it is the prime minister who is the Titan. A prime minister can, for instance, theoretically take Britain into war without either a vote in the House of Commons or a meeting of the Cabinet. Perhaps that is why it is possible for so many people in Britain to accept myth number three: that George W. Bush is recklessly leading America into one confrontation after another for weird personal or family reasons of his own.

When you ask certain senior British Civil Servants what they think of President Bush, they respond with a smile. It took me a while to learn how to translate that smile, but I think I understand it now. It says: “I am a professional and, while that notebook of yours is open, nothing you can say could possibly induce me to reveal my true opinion of that moron the Americans call their president.”

This personal disdain for Mr Bush undergirds some very basic illusions about how the American political system works—and why it fights, when it fights.

In the media, the president is often described as “the most powerful man in the world”. But that’s not how it feels to him. His cabinet officers and judicial nominees must all be approved by the Senate, and any one senator can delay an appointment almost indefinitely. The president’s budget is a mere suggestion that Congress rewrites at its pleasure.

Nor do presidents control their political organisations: four of the past seven presidents–George Bush Snr, Jimmy Carter, Gerald Ford and Lyndon Johnson—failed to win re-election after a primary challenge from within their own parties.

Presidents have to worry about holding their cabinets together. If Jack Straw were to resign tomorrow—say, to protest about some action of Tony Blair’s—it would be a 24-hour news story that would end with most journalists shrugging their shoulders and saying: “Poor guy gave up a good job.”

If Colin Powell or Donald Rumsfeld resigned, however, they would tear the Bush Administration apart, as Cyrus Vance tore apart the Carter administration when he resigned to protest at the rescue mission in Iraq in 1980.

One clever British observer said to me: “Your system works the way ours did 150 years ago.” What he meant was that the American cabinet—made up from the president’s party and from people with strong independent power-bases in the country—looks a lot more like one of Disraeli’s or Gladstone’s cabinets than it does like the party council that surrounds the contemporary all-powerful prime minister. My informant may have overstated the case a little, but only a little.

Keep those facts in mind the next time somebody suggests, as so many British journalists suggested to me, that America’s confrontation with Saddam Hussein is nothing more than the working out of a Bush family vendetta.

I’ll concede that, like the others, this myth also contains its particle of truth. It is true that Saddam attempted to assassinate the first President Bush in 1993. It is true, too, that many Republicans criticised President Clinton for his weak response to the murder plot (he fired cruise missiles at the headquarters of the Iraqi secret police after regular working hours, so fewer people would be hurt).

And it’s true that Republicans generally agree that the decision to leave Saddam in power in 1991 was the gravest error of the presidency of George Bush, one that they have a special responsibility to rectify.

But the idea that an outburst of family pique and pride can move the gigantic and sluggish American democracy to the edge of war is simply—why be polite?—nuts.

A president cannot take America into a major war all by himself. He needs the support of both houses of Congress. In 1991, after the Iraqi invasion of Kuwait, the elder Bush managed to win only 250 votes in the House of Representatives for a war resolution, and only 52 votes in the Senate. Earlier this month, the younger Bush’s Iraq resolution passed the House with 296 votes and the Senate with 75.

Are all of those 371 legislators driven by family pride? Hardly. Bush won strong congressional backing for his resolution because, since September 11, a wide consensus has been growing in America that Saddam cannot safely be left in power.

Here, for example, is Senator Joseph Lieberman, Al Gore’s running-mate in the 2000 election, in a speech delivered last year: America, he said, must be “unflinching in our determination to remove Saddam Hussein from power in Iraq before he, emboldened by September 11, strikes at us with weapons of mass destruction”. And here is Bush’s one-time rival, Senator John McCain, after the Iraq resolution debate earlier this month: “Saddam Hussein’s regime cannot be contained, deterred or accommodated.”

At bottom, the idea that Bush’s Iraq policy is inspired by personal psychological issues is based on a failure to understand how American foreign policy is made. The American government is a gigantic, messy organisation. The line between where the government stops and where the rest of society begins is never entirely clear.

Washington is full of people such as Leon Feurth, Al Gore’s former chief adviser on security issues, who have rotated out of government with their heads full of secrets—but who no longer draw a government salary; or Jim Hoagland of the Washington Post, a journalist so connected to the intelligence services that reading him is like listening to the CIA talking to itself; or Richard Perle, the former Reagan defence aide who trained an entire generation of Republican national security operatives who still look to him for ideas and advice.

These people talk to one another and argue and attend conferences together and read each other’s newspaper columns—and out of it all, ideas get hammered out and party positions are formed. And not just party positions, but true national consensus. The definitive case for war with Iraq has just been published, not by some still-bitter alumnus of the Gulf war, but by Kenneth Pollack, who analysed Iraq on President Clinton’s National Security Council.

Compared to the British parliamentary system, America’s congressional-presidential system has many disadvantages. It is slow; it often produces sloppy laws and muddled compromises. But it has one great advantage: while Parliament is organised to produce the sharpest possible divisions between the Government and the Opposition, Congress and the presidency are organised to produce something approaching a national consensus on the most important issues.

In 1993, Democratic Senator Daniel Patrick Moynihan warned President Clinton against trying to ram his healthcare proposals through Congress on a party-line vote. “Anything that big and important,” Moynihan predicted, “will pass the Senate 70-30–or not at all.” And what is true for important peacetime measures is doubly true for war.

Nothing will destroy the legitimacy of a presidency faster than any credible hint that the president is risking the lives of American servicemen for personal advantage.

Whatever small credibility President Clinton had among Republicans, he lost by ordering a missile attack on Sudan three days after his grand jury testimony in August 1998—and then approving Operation Desert Fox to begin two days before the House of Representatives’ impeachment vote in December 1998. These were the episodes that convinced even so gentle a conservative as former Reagan speechwriter Peggy Noonan that Clinton was “a bad man, and not a patriot”.

The jokey way that American columnists such as Maureen Dowd insinuate that Bush is motivated by family pride is the surest evidence that they do not believe their own accusation—for if they did, it would be no joking matter at all.

The biographer of the great New York highway builder Robert Moses tells this story. Before the First World War, Moses had unwisely made an enemy of the young Franklin Roosevelt by routing a road away from FDR’s state assembly district.

A quarter-century passed—and now Moses wanted to build a great bridge across New York harbour and name it after himself. But because the bridge crossed the route from the ocean to the Brooklyn Navy Yard, Moses needed a waiver from the Navy Department.

No problem: the bridge was more than high enough to let super-battleships sail beneath it. Or there was no problem until FDR heard about Moses’s request. He gleefully ordered the waiver to be denied.

But the matter did not stop there. The New York building unions wanted the bridge built. So did the steel and concrete manufacturers. So did the Brooklyn real-estate industry. So did the entire New York congressional delegation.

A trusted aide broke the news to the president: he would have to give way. “Are you telling me,” Roosevelt complained, “that the President of the United States cannot be allowed one teeny-weeny, little, personal animosity?” “No sir,” came the answer. “Not even one.”

At its best, the American political system is a great machine for the production of consensus. It has arrived at just such a consensus on Iraq. This conflict is not a matter of personal pique: it is an assertion of a broad national interest by a united people.