In 1653 the Cossacks allied themselves with Russia, and in turn, Russian forces invaded the Commonwealth from the east. In March that year Janusz received the office of voivode of Wilno. On 17 June 1654 Janusz was elevated from the Field Hetman position to the Grand Hetman of Lithuania. Despite the nominations, relations between him and the king have been worsening. In late August 1654 Janusz Radziwiłł defeated invading Russians at the Battle of Szkłów, but this was his last victory. Days later he was defeated by the Russians at the Battle of Szepielewicze. The following winter Lithuanian counter-offensive proved unsuccessful; with the inconclusive Siege of Mahylyow.