The First Bishops' War, fought in 1639, was the opening engagement of what would become the Wars of the Three Kingdoms — a sprawling, twelve-year disaster that engulfed Scotland, England, and Ireland. Charles I had tried to impose Anglican bishops on the Scottish kirk; the National Covenant of 1638 had refused. He decided to settle the question by force, ordering an English army of twenty thousand to march on Edinburgh, an amphibious force of five thousand to land in Aberdeen, and an Irish force under the Earl of Antrim to invade from Carrickfergus. None of it really worked. The Scots took Dumbarton before the Irish could sail. Montrose occupied Aberdeen in March, leaving the Marquis of Hamilton stuck offshore unable to disembark his poorly trained troops. The war fizzled almost before it began. The Brig of Dee was the only fight of any size.

After a failed Royalist offensive at Megray Hill on 15 June, Aboyne retreated into Aberdeen with most of his army stranded on the wrong side of the swollen River Dee. Montrose's Covenanter force lay between him and his reinforcements. In Aberdeen, Aboyne could field about a hundred and eighty cavalry and even fewer infantry. He posted a hundred musketeers under Lieutenant Colonel Johnstone on the bridge itself, reinforced the southern gate with an earthwork of 'turfs and earth, as much as the shortness of the time would permit,' and stationed his cavalry at the bridge to discourage anyone who got past the muskets. For two days, Montrose's artillery hammered the barricade. The earthwork held. The musketeers held. Then Montrose tried something different.

On 19 June, Montrose detached a troop of cavalry and sent them up the river, making a show of looking for a place to ford. The Dee was still impassable — Montrose knew that, and his cavalry knew it — but Aboyne, watching from the bridge, did not. He took the bait. He pulled his entire cavalry off the bridge to counter the supposed crossing. The moment they moved, the Covenanters renewed their assault on the southern gate. A shrapnel burst wounded Johnstone, the musketeer commander. He and his men withdrew. The barricade fell. The bridge was taken. Total casualties for both sides came to around fourteen dead apiece — a tiny number for a battle that decided the fate of a city. Aboyne, watching the bridge fall from across the river, withdrew his remaining men from Aberdeen towards Strathbogie.

Montrose's Covenanters entered Aberdeen the next day. The city remained sympathetic to the beaten Royalists, and some of Montrose's officers proposed razing it as punishment. Montrose refused. Whatever else one says about him — and his life is full of difficult things to say — he kept his men from sacking the city that he had just won. The day after his victory, news reached him that the Treaty of Berwick had been agreed. The First Bishops' War was already over. The men who had died at the bridge had been fighting a war that, by the standards of the post horse and the messenger, no longer existed. Montrose himself would change sides within a few years, becoming the most brilliant Royalist commander Scotland produced, and would be executed in 1650. The Brig of Dee still carries traffic across the river. Until 1832, it was the only way into Aberdeen from the south.