On 15 June 1940, even before the Soviet ultimatum to Latvia, Soviet special-purpose units attacked Latvian border guard posts at Masļenki and Šmaiļi in Abrene county near the Ludza River. Three border guards and two civilians were killed, several people were wounded, and 37 persons were forcibly taken across the border into the USSR. The Masļenki tragedy became a bloody prelude to the occupation of Latvia, which followed on 17 June.

The events of June 1940 unfolded after the secret protocols of the Molotov–Ribbentrop Pact had placed the Baltic states within the Soviet sphere of influence. The Masļenki attack showed that political pressure was accompanied by direct military violence against Latvia’s state border and its defenders.