As the United States celebrates the 250th anniversary of its de jure independence from the British Crown (its de facto separation came later, after years of bloody struggle), it is fitting that a scarce copy of the republic’s founding document has turned up in the National Archives in Kew. A constitutional instrument of world-historical importance, enduring bedrock of the American political settlement as it is, the declaration is also the ultimate Dear John letter, written by a partner to say things are not working out. Except in this case, the sentiment was more “it’s not me, it’s you”, rather than the usual pill-sugaring opposite.

A Dear John recipient is not supposed to file away the bad news. Indeed, the traditional response is hot-headed destruction of the offending missive followed by self-pity and revenge. In this case, however, the spurned colonial power, busy fighting what proved an unwinnable war, tucked the incendiary paperwork inside the correspondence of the Royal Navy captain who captured it along with an American ship … and promptly forgot about it for the next two and a half centuries, the duration of US history so far. A volunteer researcher came across the document by chance this year.

About 200 copies of the initial Declaration, of which 26 survive, were printed in Philadelphia. More were then made throughout the colonies for use as a recruiting tool. The Kew document is one of 11 surviving from New Hampshire. As break-up announcements go, the rebels were unsparing of their ink in getting their message across. That message being, as the soon-to-be married Taylor Swift might have put it: “We are never ever getting back together.” And yet, when judged against more venerable nations, the US is a mere adolescent. And adolescents, upon reflection in full maturity, often change their minds...