As the Tsardom of Russia went through its "Time of Troubles," Poland failed to capitalize on the situation. Military campaigns undertaken brought Poland at times close to a conquest of Russia and the Baltic coast during the Time of Troubles and False Dimitris, but military burden imposed by the ongoing rivalry also along other frontiers (the Ottoman Empire and Sweden) prevented this from being accomplished. After prolonged war with Russia, Polish forces occupied Moscow in 1610. The office of tsar, then vacant in Russia, was offered to Sigismund's son, Władysław. Sigismund, however, opposed his son's accession as tsar, as he hoped to obtain the Russian throne for himself. Two years later the Poles were driven out of Moscow and Poland lost an opportunity for a Polish-Russian union.