On December 2nd 1823 US President James Monroe officially announced what would become known as the Monroe Doctrine.

The Monroe Doctrine, in response to the various Portuguese and Spanish colonies in the Americas successfully becoming independent, stated that the New World was no longer subject to colonisation by the various European powers. Although the US would remain neutral in European matters, they would not permit interference in the Americas by the past, present and potential future colonial powers.

At the time the Doctrine was issued, the US had absolutely no means of backing it up, lacking both a credible navy and an army. So it would have been completely ignored - if it wasn't for one thing. The doctrine was supported by the British Empire, who felt it was in their best interests (and had actually been in discussion with the US regarding it) and were fully prepared to enforce it using the Royal Navy. Consequently, the doctrine turned out to be rather more effective than the other powers had originally believed.

This may have been the first, but it certainly wasn't the last, that the US and Britain collaborated on matters that were mutually beneficial.

Image: Samuel Morse [Public domain], via Wikimedia Commons (commons.wikimedia.org/wiki/File:James_Monroe_White_House_portrait_1819.gif)