US President Barack Obama and his NATO allies open a two-day summit on Friday to back a 2014 target for ceding control over the bogged-down Afghan war to Kabul.
After nine years of war provoked by Al-Qaeda's 9/11 attack, 2,200 troops killed among US-led forces, and now an open row with Kabul over tactics, NATO will deliver the closest thing yet to a withdrawal timetable.
Leaders, meeting at a complex of white steel, concrete and canvas-roofed buildings at the shores of the River Tejo, will be drawing up nothing less than the outlines of a new era for the 28-nation alliance.
The priorities for the meeting:
-- Agreement that from early next year, troops will start gradually returning home. By 2014, the goal is for Afghans to control most of the battlefield, disputed by a fierce Taliban resistance.
-- Erecting an anti-ballistic missile shield across the skies of Europe to protect NATO members in the continent, and overcoming Russian fears by inviting them to take part.
-- Reform the alliance to slash the number of command headquarters and make them more easily deployable to faraway conflicts such as Afghanistan.
-- Unveil a new "strategic concept", NATO's mission statement for the next decade to counter 21st century threats, from cyber strikes to global terrorism.
Navigating a way out of the Afghan war, the biggest and longest military operation undertaken by NATO since its creation in 1949 to contain the Soviet Union, is the biggest challenge in Lisbon.
NATO Secretary General Anders Fogh Rasmussen admitted on the eve of the summit that the alliance had made mistakes in waging the war, notably by waiting until last year to set up a training mission.
"I think, retrospectively, that we underestimated the challenge and our operation in Afghanistan didn't have sufficient resources, so in that respect, yes, it was a mistake," Rasmussen told Portuguese newspaper Expresso.
"Now we are on the right path and it is for that reason that I am quite optimistic about our operation in Afghanistan, and we will make this positive announcement in Lisbon, that the transition is ready to start," he said.
NATO officials insist that the transition is not a rush to the exit door, but the war is deeply unpopular in Europe and governments are under pressure from voters to bring soldiers home.
However, Obama still faces resistance from his own high command and from newly resurgent Republicans in Congress.
The Pentagon said on Thursday the plan to hand over security to Afghan forces in 2014 represents an "aspirational goal" and not a rigid deadline.
Press secretary Geoff Morrell said there "may very well be the need for (US-led) forces to remain in-country" beyond 2014, albeit in smaller numbers.
That echoed remarks on Wednesday by NATO's civilian representative in Afghanistan, Mark Sedwill.
The date was chosen by Afghan President Hamid Karzai, who shocked NATO allies this week by urging the United States to scale down military operations and criticising night raids on Afghan homes.
US Defence Secretary Robert Gates sought to play down any rift, saying on Tuesday that Karzai's comments reflected "the impatience of a country that's been at war for 30 years".
Karzai, who arrived in Lisbon late Thursday, was to meet with Obama and other Western leaders on the sidelines of the summit, which was being held under tight security.
Hundreds of police closed off and secured streets surrounding the complex. A frigate patrolled the river. Security forces looked down with binoculars from the city's Vasco da Gama Tower.
Inside, NATO leaders will take steps to protect Europe from ballistic missiles by agreeing to link up defence systems, although they will not name Iran as the main threat in order to secure Turkey's backing for the project.
At the same time, the Western military club will invite its former Cold War foe Russia to come under the umbrella as part of efforts to improve relations that were dramatically strained by Russia's war with Georgia in 2008.