Together with Bogusław Radziwiłł, Janusz offered his protection and a refuge to Calvinists in Lithuania. As field hetman he was placed under pressure in 1648 by an uprising of the Ukrainian people led by Bohdan Khmelnytsky, who was defeated by Muscovite forces in 1655. In the same year he allied himself with the Swedes and recognised Charles X Gusta as king.
Radziwiłł was interested in Comenius’ pansophic work and invited him to Lithuania, offering him advantageous working conditions.