The North Atlantic Treaty Organisation (Nato) is the largest military force ever assembled, with a potential armed force of more than seven million.
Two decades after its original enemy, the Soviet Union, disintegrated, the alliance has been searching for a new identity and new role.
As it reinvents itself for the 21st century, Nato is also redefining its threat assessments and its area of operations.
Its philosophy is summed up in Article 5 of the treaty, all for one, and one for all. But does this 'Three Musketeers' approach work when there are 28 different or conflicting agendas?
Is this alliance anything more than a fig leaf for US foreign policy? And what sort of relationship can it strike with reinvigorated Russia?
Nato's new strategic concept unveiled in Lisbon underlines the global role of the alliance's new non-continental or non-geographic preoccupations such as cyber and space.
Is this the way to stay relevant in a complicated world? What does the future hold for Nato in Afghanistan and beyond?
Empire finds out.
Joining the programme: Anders Fogh Rasmussen, Nato secretary general; General Sir Michael Jackson, former commander, Nato Allied Corps; Oksana Antonenko, Russia & Eurasia director, IISS; Dr Gülnur Aybet, International Relations, Kent University; William Pfaff, author of The Irony Of Manifest Destiny; Professor Stephen M Walt, international affairs, Harvard University; Diana Johnstone, author of Fools Crusade; Dr Vlad Sobell, Daiwa Research Institute; Professor Alex Callinicos, author of Bonfire Of Illusions.