The Auld Alliance

In 1295, as a result of English aggression, John Balliol of Scotland and Philippe IV of France signed the Treaty of Paris which formed the a military alliance between the two countries (called the "Auld/Vieille Alliance" following the British Union of the Crowns in 1603). During the Hundred Year's War, the French carried flags with a white cross on either a blue or red background onto the battlefield to distinguish themselves from their Anglo-Burgundian adversaries, both of whom used red crosses. It is during this conflict that the Auld Alliance made its presence felt: a thousand Scots served under Jean d'Arc at Orléans alone.

In 1558, the heirs of the two kingdoms - Mary I Stuart of Scotland and the future François II Valois of France - were wed (though the marriage would end two years later with François' death, OTL). Had the union produced children, however, it would have united not only the thrones of France and Scotland, but potentially England as well.