Watch part two

Widely written off as a relic from the Cold War a few years ago, Nato found a new purpose on the international arena following the 9/11 attacks in New York eight years ago.

The alliance of the US, Canada and 28 European countries, invoked article 5 of its charter for the first time in its 60-year history following the attacks. The article says that an attack on any member shall be considered an attack on all.

Two years later, Nato took control of the International Security Assistance Force, which commands about 55,000 troops in Afghanistan.

But as the Afghan War drags on, and casualties from member states rise, many Nato countries want to see a timeline for a conclusion of the conflict.

Meanwhile, tensions with Moscow continue to grow as Nato expands its membership deeper into former Warsaw Pact territory.

What are the challenges laying ahead for the world's biggest military alliance?

In New York for a special series of programmes as world leaders gather for the UN general assembly Riz meets veteran Danish politician Anders Fogh Rasmussen, the new secretary-general of Nato.