Since Valdemar IV had also attacked ships of the Dutch city of Kampen and other destinations in the Zuiderzee, Prussia and Dutch cities, such as Kampen, Elburg and Harderwijk, allied themselves against Denmark. This then made the Hansa calling up a diet in Cologne in 1367, also convening the afore-mentioned and more non-member cities like Amsterdam and Brielle, founding the Cologne Federation as a war alliance, in order to ban the Danish threat. More cities from the Lower Rhine area till up to Livonia joined.