With the war against Russia still ongoing, the Commonwealth suddenly had to face a new enemy. In June 1655 Swedish forces started to advance across the northern territories of the Commonwealth. The Swedish invasion of Polish–Lithuanian Commonwealth, part of the Second Northern War, is known in the Polish history as The Deluge. Together with his cousin Bogusław Radziwiłł, Janusz began talks with Swedish king Charles X Gustav of Sweden, discussing how to reverse the Union of Lublin which created the Commonwealth. Janusz first declared the Grand Duchy a Swedish protectorate in the Treaty of Kėdainiai on 17 August 1655, then the brothers signed another treaty on 20 October according to which the Swedish–Lithuanian union was founded, and in which Radziwiłł's were to rule a part of the Grand Duchy.