NATO Summit: Leaders gather in London to mark the 70th anniversary

Leaders gather in London to mark the 70th anniversary NATO leaders will gather on Tuesday in the UK to mark the 70th anniversary of the military alliance. Since it was formed, it's expanded to include 29 countries and take on missions far beyond Europe. But recent years have seen growing internal tension, and new global threats. Simon McGregor-Wood looks back at the history and some of the challenges members are facing in what could be a tense meeting.