The Verden Insurrection, as the name suggests, began in Verden in 1267 after the Nilfgaardian invasion and subjugation of the kingdom under Ervyll. At first, the insurrection counted only about five hundred people in its ranks; those outraged by the brutality of the Verden Operations Group as they invaded. They were not only cruel in battle, but inflicted the same wanton cruelty on the general population as well.
Initially Ervyll put up some token resistance (but this only prompted further bloodshed), and he was seen as a Nilfgaardian sympathizer and ultimately assassinated. His son, prince Kistrin took over as leader of the resistance. The corsairs from Skellige and the troops from Cidaris helped them to expel Joachim de Wett's group from Verden, but only due to de Wett's gross incompetence.
The rebellion was partially suppressed in 1268 by the Center Army Group led by Field Marshal Menno Coehoorn. The rebellion however slowed Nilfgaardian troops from reaching Temeria, thus giving the Nordlings extra time for preparations for the Battle of Brenna.
The final nail in the coffin for the Verden Operations Group was exactly the disaster at Brenna when Joachim and his troops were decimated by the north under Foltest and John Natalis.
After their defeat, Nilfgaard was forced to withdraw their armies to south of the Jaruga, leaving the only the fortresses of Rozrog, Nastrog and Bodrog occupied. After the peace treaty, the rest of the Nilfgaardians left and Verden was finally liberated, with Kistrin as the new ruler.