British Prime Minister Tony Blair speaks to a joint session of Congress Thursday at a moment when the "special relationship" between the US and Britain is under particular strain. [Editor's note: The original version of this story misstated the timing of an honor Congress intends to bestow upon British Prime Minister Tony Blair.]
The primary reason is the continuing controversy over intelligence cited by Mr. Blair and President Bush to justify the Iraq war. The US has tapped Britain as the source of now-discredited information, while British officials say the US held back key information on the intelligence in the run-up to war.
Beyond that, an unstable and deadly postwar Iraq is replacing what was supposed to be a victory glow with an uncomfortable spotlight on cracks in the tight US-British friendship. Bush can hardly relish the focus Blair's visit places on the intelligence controversy. Blair, meanwhile - already under attack at home - will be looking for help from his friend George while seeking to show constituents there is daylight between the two countries.
"Blair really is coming at the worst possible moment for both leaders," says Ivo Daalder, an expert in US-Europe relations at Washington's Brookings Institution. "[His] medal is lost in the maelstrom of controversy about the reasons for going to war and the aftermath."
The visit may give US and British audiences a stronger hint of what separates the two countries, as well as what unites them, than at any time in the recent past.
The two have marched side by side in Afghanistan and Iraq, speak as one on North Korea, and sing from the same hymn sheet where trouble spots like Zimbabwe and Colombia are concerned - leading some of Blair's detractors to complain that Britain and America are now indistinguishable on matters of foreign policy.
They have dubbed Blair "US foreign minister" and Bush's "poodle," while some international critics say London has lost its independent voice and is merely doing Washington's bidding.
But Britain and America have numerous issues on which they scarcely see eye to eye - and the list appears to be growing.
At the top currently is America's plan to try suspected terrorists, including some Britons, at Guantanamo. Differences on Iran and the Middle East also lurk just below the surface, as do disagreements on trade, global warming, genetically modified crops, the International Criminal Court, and arms control.
But where other powers might choose to fall out or lock horns over such matters, Britain is trying to influence its powerful friend by standing by it. The approach has brought Blair important trophies, some experts say.
"The 'poodle' comment we hear so often is belied by the reality of how diplomacy works, and in particular by what Blair has to show for his regularly whispering in the ear of a partner he doesn't always agree with," says John Hulsman, a European specialist at the Heritage Foundation in Washington.
Among Blair's "tangible results," he includes the fact that the US went through the UN at all before going to war in Iraq; the administration's "helpful" engagement on Northern Ireland; and getting Bush to push the road map for Middle East peace.
"The fact is, Blair is among the president's top three to four advisers on foreign affairs," Mr. Hulsman says.
Blair's emphasis on the "special relationship" is of course not new - though the political trouble he's now facing, in part because of it, is particularly intense. "There has been a very long history of Britain playing Greece to America's Rome," says Barry Buzan, professor of international relations at the London School of Economics. "It has been the British position for several decades now, to be the bridge to North America and at the same time having to avoid choosing sides," he adds.
Blair is no exception, Mr. Daalder notes, despite being more pro-European than most of his predecessors. "There are huge differences between Blair's foreign policy and Bush's," he says. "Blair has a very different view of the nature of the world in which we live than Bush. But when push comes to shove, and the US goes one way, Britain can oppose or join in and mold the policy direction of the US."
One example is Iran. Britain disagrees with the idea of regime change, while quietly representing the "Anglo-Saxon" viewpoint in regular ministerial forays to Tehran. "We try very much to play the role of honest broker between the American view and other people's views," says one British diplomat who has worked alongside US counterparts in the UN and Middle East. "Lots of countries wouldn't think too highly of the Americans but are happier to speak to us; we can help the Americans by representing their views."
Still, as Blair visits Congress and the White House, the question remains whether Blair's influence is enough to allow him to claim success in his strategy.
When Blair and Bush first met at Camp David in February 2001, Bush said pointedly, "I can assure you that when either of us gets in a bind, there will be a friend on the other end of the phone."
With his domestic prospects sagging, Blair could use some help from Bush - an agreement to extradite two British nationals being held in Guantanamo, for example, or hints at a better shake for British companies in Iraq's reconstruction.
"It has not been well understood on this side of the Atlantic how much trouble Blair has amassed for his position on Iraq, or to what extent the British support on Iraq was just Blair's and not from the British public," says Klaus Larres, a visiting scholar at the Library of Congress from the University of London.
Blair said earlier this week that he expects one or two quid pro quos from Bush. While noting that the US has genuine concerns about terrorism and rogue states, he said Britain in return wants action on such issues as global poverty, trade, climate change, and the Middle East.
Daalder says Blair will realize that he "is not able sufficiently to mold US policy in the direction he wants" - and that whatever Bush offers is unlikely to sway a British public upset by broader domestic problems.
Still, the two leaders will stick together because they need each other, Mr. Larres says. "For either one to switch course when they've gone this far together would only make things worse," he says.