Under Stalinist rule, populations who were not Ukrainian, Russian, or Romanian were pressured to russify, and their numbers would decline further. After a brief initial period of liberalization and freedom, groups such as the Poles in the Soviet Union were subject to harassment, dispersal and mass terror. This trend increased in the late 1930s, as a result of the 1937-8 Polish Operation of the NKVD as well as the ceasing of educational instruction in the Moldavian ASSR for all non-Romanians populations in their native languages which was replaced by Ukrainian and Russian.