George Marshall, two diplomatic messages printed on blue paper to emphasise their importance: one on Greece, the other on Turkey.

Exhausted, broke and heavily in debt to the United States, Britain told the US that it could no longer continue its support for the Greek government forces that were fighting an armed Communist insurgency. Britain had already announced plans to pull out of Palestine and India and to wind down its presence in Egypt.

The United States saw immediately that there was now a real danger that Greece would fall to the Communists and, by extension, to Soviet control. And if Greece went, the United States feared that Turkey could be next, giving Moscow control of the Eastern Mediterranean including, potentially, the Suez Canal, a vital global trade route. Almost overnight, the United States stepped into the vacuum left by the departing British.

"It must be a policy of the United States," President Harry Truman announced, "to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressure."

It was the start of what became known as the Truman Doctrine. At its heart was the idea that helping to defend democracy abroad was vital to the United States' national interests.

There followed two major US initiatives: the Marshall Plan, a massive package of assistance to rebuild the shattered economies of Europe, and the creation of Nato in 1949, which was designed to defend democracies from a Soviet Union that had now extended its control over the eastern part of Europe.

It is easy to see this as the moment that leadership of the western world passed from Britain to the United States. More accurately it is the moment that revealed that it already had. The United States, traditionally isolationist and safely sheltered by two vast oceans, had emerged from World War Two as the leader of the free world. As America projected its power around the globe, it spent the post-war decades remaking much of the world in its own image. The baby boomer generation grew up in a world that looked, sounded and behaved more like the United States than ever before. And it became the western world's cultural, economic and military hegemon. Yet the fundamental assumptions on which the United States has based its geostrategic ambitions now look set to change. Donald Trump is the first US President since World War Two to challenge the role that his country set for itself many decades ago. And he is doing this in such a way that, to many, the old world order appears to be over - and the new world order has yet to take shape. (BBC/Getty Images)