I doubt whether Britain can be classed as a global superpower in 1939. It had global commitments but without the resources to fulfil them. This was the dilemma for the British during the 1930s. When Britain declared war on Germany in 1939, the Treasury said Britain could only afford to wage a short war; the Chiefs of Staff said Britain could only hope to win a long one.

Britain's economic decline meant that it had an insufficient manufacturing base to produce the armaments needed to defeat Germany. It therefore purchased armaments from America, which had a much larger manufacturing capacity. In March 1941, as Britain's resources (dollar and gold reserves) to pay for American armaments were nearing exhaustion, America decided to prop up Britain with Lend-Lease. This meant Britain could borrow armaments from America to fight the Axis. In fact the British war effort after 1941 was an expression of American power as it was only possible through American help. By 1945, Britain was massively in debt.

The war exposed British weakness by demonstrating that Britain no longer had the economic or military resources by itself to maintain its position. However the loss of superpower status was not caused by the war, it was caused by Britain's inefficient and backward industrial base that could not compete with Germany and America.