In June 1940 he received the rank of major general and was appointed deputy commander of the 15th Army of the Far Eastern Front.

Lopatin was transferred west in November 1940 to command the 31st Rifle Corps of the Kiev Special Military District.

During June and July 1942 he commanded the 9th Army of the Southwestern Front, which stopped the German advance in the Donbass and the Don river bend.

[1] In January 1944 Lopatin was reduced to deputy commander of the 43rd Army of the 1st Baltic Front, which took part in Operation Bagration.

In July he was appointed commander of the army's 13th Guards Rifle Corps at his own request, which took part in the advance into the Baltics and the East Prussian offensive.

The corps ended the war in the Samland offensive, overcoming German fortifications and capturing the cities of Gross-Haidenburg, Zimmerbude, and Payze.

[1] Lopatin was sent to the Far East in preparation for the Soviet invasion of Manchuria, taking command of the 2nd Separate Rifle Corps in July 1945.