When in 1648 the Peace of Westphalia ended the Thirty Years' War, the parties agreed that the prince-bishoprics of Bremen and Verden were to become dominions of Sweden. The peace treaty had been prepared at a congress throughout the final years of the war. After the war a dispute about custom collection between Bremen and the emperor led to the latter imposing an Imperial ban on the city in 1652/53. Sweden, which had never accepted Bremen's Imperial immediacy, attacked in 1654.