David Cameron is flying to the United States for talks with President Barack Obama at which the two leaders will discuss the timetable for withdrawal of allied troops from Afghanistan.
Both men 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 is some speculation that this could be brought forward to the summer of next year.
Mr 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.
Mr Cameron and Mr Obama used a joint article in the Washington Post 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.
Mr 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 to hail the "essential relationship" between their two nations.
Labour said the Government appeared to have an exit date for Afghanistan but no exit strategy.
Shadow defence secretary Jim Murphy said: "It is unsettling for our forces and their families to read about new plans for withdrawal in the media. The Government appears to have an exit date but not yet an exit strategy. There is more to leaving Afghanistan than choosing a date."