At the Treaty of San Ildefonso, Andrew Roberts explains (in Napoleon: A Life), Napoleon had promised Spain not to sell Louisiana to a third party, a commitment he decided to ignore:
On the same day that Whitworth called for his passports in Paris, across the Atlantic President Thomas Jefferson signed the Louisiana Purchase, doubling the size of the United States at the stroke of his pen. The Americans paid France 80 million francs for 875,000 square miles of territory that today comprises all or some of thirteen states from the Gulf of Mexico across the Midwest right up to the Canadian border, at a cost of less than four cents an acre.
[…]
‘I have just given to England a maritime rival that sooner or later will humble her pride.’ Within a decade, the United States was at war with Britain rather than with France, and the War of 1812 was to draw off British forces that were still fighting in February 1815, and which might otherwise have been present at Waterloo.
[…]
The financing was arranged via the Anglo-Dutch merchant banks Barings Brothers and Hopes, which in effect bought Louisiana from France and sold it on to the United States for $11.25 million of 6 per cent American bonds, meaning that the American government did not have to provide the capital immediately. As a result, Barings were paying Napoleon 2 million francs a month even when Britain was at war with France. When the prime minister, Henry Addington, asked the bank to cease the remittances Barings agreed, but Hopes, based on the continent, continued to pay and were backed by Barings – so Napoleon got his money and Barings and Hopes made nearly $3 million from the deal.
So which big Continental banks have sent real estate appraisers to poke around Greenland this month?