Hitler's invasion of Poland in September 1939 precipitated the commencement of WW2 in Europe. Even before America was drawn into the war by the Japanese attack on Pearl Harbour in December 1941, it was already entangled via the Lend-Lease programme, through which it was providing material and financial support to a cash-strapped Britain. Churchill described America's Lend-Lease programme as a ‘most unsordid act’,ssss1 but it was also one of calculated self-interest since, if Britain were to be defeated by Germany, the US would be left standing alone against a fascist-controlled Europe. For Britain, however, it meant it would face a costly debt to the US after the war.

Keynes returned to the Treasury as an unpaid advisor to the Chancellor of the Exchequer after WW2 started. By this time, he was becoming a part of the establishment that he had previously derided. He was elected to the Court of the Bank of England in 1941 and then ennobled as Baron Keynes of Tilton the following year. Understanding the implications of his country's financial position and the need to plan ahead for its changed circumstances after the war, he began work in August 1941 on a plan for a new post-war global monetary order.