The Crazy Irony!
Napoleon Bonaparte, desperately needed cash to fund his wars in Europe. His colonies in the Americas, especially Louisiana, were becoming a burden — hard to defend and not making enough money.
So he sold Louisiana to the USA for $15 million. But the US government didn’t have that much ready cash lying around. So they turned to big British and Dutch banks (Barings Bank in London and Hope & Co in Amsterdam) to finance the deal.
These banks bought US government bonds — meaning British investors’ money was used to pay France. Napoleon grabbed that cash to build ships, train troops, and prepare for an invasion of Britain.
So, the British banks ended up funding Napoleon’s army that was getting ready to invade Britain itself!