In 1672 the Dutch Republic was attacked from three sides at once. France invaded from the south, England struck at sea, and from the east came Bernhard von Galen and Maximilian Henry of Bavaria, the elector of Cologne. The French rolled up Rhine fortresses - Rheinberg, Wesel - in under a week. The bishop and the elector took Lingen, swept into Overijssel, helped the French seize Groenlo. On 19 July, Bernhard captured Coevorden with an army of 24,000 men. He then faced a choice: invade Friesland, or turn on Groningen. He chose Groningen. The connection to his base at Nieuweschans would be closer, and he calculated Friesland could be cracked open later with a few thousand troops. He arrived before Groningen on the same day Coevorden fell.

What Bernhard had not counted on was Carl von Rabenhaupt. The Bohemian-born commander of the Groningen garrison had spent the weeks before the siege preparing the city's defences with around 2,500 men. When the bombardment opened on 21 July, Groningen did not collapse the way the smaller towns had. The countryside outside the walls was plundered and houses around the city were destroyed. On 27 July, the city sent word to the States-General begging for reinforcements. William III of Orange agreed to send a regiment. Meanwhile in the marshes of Friesland, a Frisian noble named Hans Willem van Aylva was running a guerrilla campaign against Bernhard's supply lines - and the supplies of an attacking army are what decide a siege.

Bernhard's bombardment was technologically modern for its day. Cannon firing actual explosive bombs - not just solid shot - hammered the city walls and the streets behind them. This was what earned him his Dutch nickname. But for all the noise, the bombs were not enough. The bishop's men began to starve. Aylva's raids on the supply trains, and the basic problem of feeding 24,000 soldiers in plundered countryside, broke the besiegers before they broke the city. On 27 August, Bernhard ordered the withdrawal. By the time he had pulled his force away, he had lost somewhere between five and ten thousand men. The next day - the morning of 28 August - Groningen woke up to find the besiegers gone.

The defeat at Groningen was not just a local victory. It ended the Munster offensive into the northern Netherlands. In December of the same year, Rabenhaupt marched out and recaptured Coevorden. Friesland, which Bernhard had planned to take next, was now safe - and that mattered enormously, because the Amsterdam trade routes around the Zuiderzee ran through Frisian waters. Of the three invasions of the Rampjaar, the eastern one had been turned back first. By the time the war ended, the Republic had survived in a form recognisable as itself.

Holidays that survive for centuries do so because they answer a need that does not go away. Gronings Ontzet on 28 August is partly civic pride - the city of Groningen stood when so much else fell - and partly the very Dutch instinct to turn calamity into a story you can tell at a table. The bishop got a nickname instead of a victory. The street fair, the funfair, the morning service in the Martinikerk and the firing of the cannon all knit the modern city back to an August day in 1672 when the smoke cleared and the besiegers were gone. The man who tried to burn Groningen down has been folded, gently and permanently, into Groningen's calendar.