Zasulich was a graduate of the Cadet Corps and the Konstantinovskoe Military Academy and was commissioned as a lieutenant in 1863, serving with the 93rd Infantry Regiment in Irkutsk.

He was with this regiment during the Russo-Turkish War (1877-1878), during which he was highly decorated several times for bravery in combat in the Balkans, especially during the Battle of Philippopolis (1878).

Assigned to prevent the Imperial Japanese Army from crossing the Yalu River into Manchuria, he made the tactical error of spreading his forces piecemeal over a 170-mile front and heavily fortifying the border of town of Antung, where he was convinced the main attack would come.

Assuming that the Japanese army amassing near Wiju upstream from Antung was a feint, he refused requests to redeploy his forces.

In the subsequent Battle of Yalu River, the Russian forces were routed by Japanese First Army under General Kuroki Tamemoto.