What is Nato?

Nato members agree to support each other if they come under attack. That’s the fundamental purpose of Nato, or the North Atlantic Treaty Organization, formed in the wake of World War II. But how has its remit changed in the seven decades since its foundation? Since Russia’s invasion of Ukraine, more states have sought to join Nato, but its expansion is perceived as a threat by President Putin.James Landale, the BBC's Diplomatic Correspondent, explains how Nato works and what challenges lie ahead for the organisation.