Final answer:
Nicholas II left for the front lines during World War I and Alexander Kerensky took charge of the provisional government. St. Petersburg experienced political and social unrest during Nicholas' absence.
Step-by-step explanation:
Nicholas II, the last Tsar of Russia, left for the front lines during World War I to personally command the Russian army. In his absence, he put Alexander Kerensky in charge of the provisional government. While Nicholas was away, St. Petersburg experienced political and social turmoil, including the Decembrist Uprising and the rise of radical terrorist groups.