Welcome Back. The North Atlantic Treaty Organisation - or NATO marks its 60th anniversary on Saturday. The world's most powerful military alliance was formed after the Second World War - as a counterweight to the Soviet Union and its allies. In 1955, the communist bloc came together as the Warsaw Pact. And the world was divided into two hostile camps. Much has changed since then. The Cold War is over.. and NATO expanded to include 28 members.. some of them former communist countries. Croatia and Albania became the newest to join the group earlier this week. The enlargement of NATO is viewed with some suspicion by Russia - particularly the inclusion of former Warsaw Pact countries. Just as the composition of the alliance has changed - so have the challenges which face it. In our special report today, we look back at NATO's eventful history in the past sixty years.