On 27 February 1560 in the town of Berwick, on the Anglo-Scottish border, a secret agreement was signed containing a range of unusual and profoundly important clauses. In the first place it constituted a diplomatic revolution. A group of Scots were abandoning their ‘auld alliance’ with France and embracing as allies their long-standing enemies, the English. The central purpose of the Treaty of Berwick was to furnish desperately needed English military aid to the Scottish Lords of the Congregation to prevent them being overrun by the French troops of Mary of Guise, Scotland's regent. From a Scottish perspective, the language employed to describe this diplomatic revolution was equally remarkable. Though exchanging one dominant protector for another, the treaty had carefully avoided any threat to Scotland's independence, in particular making no mention of England's imperial claims that had played such a prominent part in previous Tudor incursions across the Border.

Another unusual feature was that Elizabeth I of England concluded this formal alliance not with a fellow monarch, but rather with those Scots who were fighting against the regent and her daughter, the Scottish queen. In the volatile international situation of spring 1560, aiding rebellious subjects against their lawful rulers was a risky diplomatic gamble for the precarious Elizabethan regime. It also ran counter to all Elizabeth's political and personal instincts, yet the queen had been persuaded of its necessity by her chief adviser William Cecil, supported by her Privy Council.