These baltic states were just vassals of Germany and once were part of Russian empire till 1918 when Lenin signed the Brest treaty and gave up a chunk of lands to German protection. Western Ukraine was also taken away by Poland I guess. So in Molotov pact they just reversed it with some assurance that USSR will not attack if Nazi Germany attacks Poland . I remember I didn't know that baltic States were under Russian empire until I watched an old video of Putin angrily replying this to a Latvian journo in early 2000s. Why Baltics have strong Nazi memories is due to that only, they were nazified with German influence.