Simplify the following text for a Grade 7 native English-speaking person.
Original: The Nikolayevsk Incident was a series of events from February through March 1920 during the Allied intervention in the Russian Civil War , cumulating in the massacre of several hundred Japanese expatriates and most of the Russian inhabitants of the town of Nikolayevsk-on-Amur in the Russian Far East .
Simplified: The Nikolayevsk Incident was a series of several events during the Russian Civil War . They happened between February and March 1920 .