David Cameron flies to the USA today for talks with President Barack Obama, with the timetable for withdrawal of British and American troops from Afghanistan likely to top the agenda.
Both leaders have stressed in public that there will be no rush to the exit in the wake of the recent deaths of six British soldiers and the massacre of 16 Afghan civilians by a renegade US serviceman.
They are expected to focus during the three-day visit on the timing of handover of the lead security responsibility throughout the country to Afghan forces during 2013.
This will allow allied troops to step back into a support role in the fight against the Taliban and begin the process of returning home by the previously-agreed target of the end of 2014.
An announcement on the date for transition to Afghan control is not expected until Nato's Chicago summit in May. Nato agreed at a previous summit in Lisbon in 2010 that home-grown forces would take the lead responsibility for security by the end of 2013, but there was some speculation today that this could be brought forward to the summer of next year.
Cameron wants to ensure that the UK and US are "in lockstep" over the handling of the sensitive conclusion to an operation which will have lasted 13 years by the time international troops are pulled out.
Cameron and Mr Obama used a joint article in the Washington Post today to declare that they are "proud of the progress our troops have made in dismantling al Qaida, breaking the Taliban's momentum and training Afghan forces".
They added: "As recent days remind us, this remains a difficult mission. We honour the profound sacrifices of our forces and in their name we'll carry on the mission."
The three-day visit will also be a celebration of the close links between the US and Britain, with a star-studded state dinner at the White House, a trip on Air Force One for the two leaders to watch a basketball game in Ohio, and public appearances for Samantha Cameron with First Lady Michelle Obama.
Cameron is the first foreign leader to be invited by Mr Obama to fly with him on his official jet, and the two men used their joint newspaper article today to hail the "essential relationship" between their two nations.
"Our troops and citizens have long shown what can be achieved when British and Americans work together, heart and hand, and why this remains an essential relationship - to our nations and the world," they wrote.
"So, like generations before us, we're going to keep it up. Because with confidence in our cause and faith in each other, we still believe that there is hardly anything we cannot do."
The two leaders said they will also use the visit to discuss economic and diplomatic measures to "tighten the noose" on the regime of Syrian dictator Bashar Assad in the hope of pressuring him to end violence against opponents and step down from power. They are not, however, expected to discuss the option of sending arms to rebels.
On Iran, Cameron and Obama sent a warning to President Mahmoud Ahmadinejad to meet his international obligations to give up any ambition of a military nuclear capability or "face the consequences", with tough sanctions from both the US and EU.
But in what will be seen as a signal to Israel to show restraint, they stressed that there was still "time and space to pursue a diplomatic solution".
The so-called E3+3 process of talks is expected to restart next month after more than a year's suspension, as Tehran indicated it is ready to withdraw pre-conditions to negotiations.
The economy will also feature in the talks, even though Obama's approach to dealing with the downturn features more state stimulus and less reliance on cuts and austerity than the coalition's programme.
One "blue skies" idea which Britain will be pushing is a free trade area uniting the US and European Union, though agreement on this proposal is thought to be some way off.
Cameron's visit will inevitably be seen in the US as part of Mr Obama's campaign for re-election in November's presidential elections.
The Prime Minister will not see any of the candidates for the Republican presidential nomination during his visit, though Downing Street played down suggestions that this amounted to a snub - pointing out that none of them requested a meeting. There will be Republicans present at tomorrow's official lunch and state dinner.
For both leaders, this evening's visit to a university basketball match in Dayton, Ohio, will be a highly welcome photo-opportunity as well as a chance to get to know one another better in an informal setting.
This evening's "First Four" event marks the start of the popular NCAA March Madness tournament, which is televised to large audiences across the States.
The decision to take Cameron to the game has been panned by a Republican party looking to oust Obama from the White House in November.
"While showing off our amazing college basketball teams is great, many Americans struggling to find jobs, dealing with soaring gas prices, or concerned with our rising deficit and debt would probably like the president spend at least as much time dealing with those issues," said Sean Spicer, a spokesman for the Republican National Committee.
While Obama is a known basketball fan and has frequently shown off his skills on the court, the same can not be said for Cameron.
British Ambassador Peter Westmacott told reporters in Washington on Monday that Cameron was "busy briefing himself on March Madness."
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