The Secretary General of the North Atlantic Treaty Organization, Jaap de Hoop Scheffer is stepping down after 5.5 years heading NATO. Hoop Scheffer says his successor, former Danish Prime Minister Anders Fogh Rasmussen inherits an alliance that will need to adapt to remain relevant in the 21 century.

Afghanistan has 60,000 troops  from 15 countries, and is perhaps NATO's biggest challenge.

Secretary General Jaap de Hoop Scheffer says the alliance must stay and fight. "If we were to walk away, Afghanistan would fall to the Taliban, with devastating effect for the people there - women in particular," he said. "Pakistan would suffer the consequences, with all that that implies for international security."

Critics say NATO's performance in Afghanistan has revealed cracks in the alliance - not enough political will among members, poor logistical coordination and no cohesive command structure. NATO's difficulties in Afghanistan are not only military - but diplomatic  and practical as well. Ravaged by decades of war - the country must be rebuilt.

"True success in Afghanistan requires civil reconstruction - something which NATO cannot provide, but which others must supply.  If these other actors do not engage, NATO cannot truly succeed either," Scheffer assets. "To put it bluntly: NATO's success depends increasingly on factors which lie outside the Alliance's own control."

De Hoop Scheffer is handing the reins of NATO to former Danish Prime minister Anders Fogh Rasmussen. "I look very much forward to continuing the good job done by Secretary General Scheffer in the transformation of NATO to manage the new challenges of the 21st century," he said.

Those new challenges mean the alliance must redefine itself, says NATO analyst Alastair Cameron of the Royal United Services Institute, a think-tank here in London. "I think that the alliance will want to develop roles in areas that it previously didn't have responsibilities in, such as energy security, cyber defense, new soft elements of activity," Cameron said.

There's also the question of further expansion.  NATO's already includes 28 nations. Georgia and Ukraine would like to join. Georgia at least - has America's backing.

"We understand that Georgia aspires to join NATO," Mr. Biden said. "We fully support that aspiration and we will work to continue to help you meet the standards of NATO membership."

That has strained NATO's relationship with Russia - another challenge Mr. Rasmussen will have to tackle as the new Secretary General. One of the first things he's expected to address is constructing NATO's Strategic Concept - it's driving purpose. Some countries believe Article 5 - collective Self Defense  - should remain the  alliance's central focus.  But not all members feel the same.

"I think if NATO is to remain relevant, it needs to address the divisions within its heart," Cameron adds, "I think that Europeans amongst themselves have different perspectives of what the alliance should be there for."

There is no question that NATO's most visible challenge is Afghanistan. The outcome of the conflict will help shape  NATO's future - and the way the world's oldest alliance plans to develop in the 21st cenutry.