When did the United States replace Great Britain as the world's most powerful nation?

World War One rapidly ended the United Kingdom’s global hegemony. While Germany had already surpassed the UK industrially before the war began, Britain’s power came from its interwoven global and commercial empires.

The war was literally a negative-sum game - the most costly and unnecessary blunder a major power could have committed. The only purpose for entering into a continental conflict was to defend an old, almost forgotten, alliance with tiny Belgium. Great Britain could have waited out the whole disaster on the sidelines. Instead, by declaring war on Germany (on August 4, 1914) it escalated a European war into a world war.

The British Empire lost almost one million men, compared with the United States’ 117,000. The war cost Britain $47 billion, versus the U.S.’s $27 billion. By the end of the debacle, Britain's debt exceeded 200% of its GDP. The U.S. took over financing the Allies in 1917 with loans that it insisted had to be repaid.

On December 31, 2006, Britain made its final payment of about $83 million and thereby was discharged of the last of its WWI loans from the United States.