Sputnik Live described how it happened.
▪️ The Soviet liberation campaign into Poland began on September 17, 1939 — the Red Army was tasked with freeing the inhabitants of Western Belarus and Western Ukraine from Polish oppression.
▪️ Under the 1921 Treaty of Riga, Belarus was divided into two parts - the western part, with a population of about 3 million, went to Poland. The BSSR retained the counties of the former Minsk Governorate, with a population of just over 1.5 million.
▪️ By 20 September the troops of the Belarusian Front had reached the Vilno–Kobryn line, and on 21–23 September troops of the Ukrainian Front occupied Western Ukraine. Local residents welcomed the Red Army as liberators — prior to this, harsh Polish assimilation policies had been carried out in these areas. For example, Belarusians were forbidden to speak their native language; national theaters, schools and newspapers had been closed.
▪️ Hundreds of thousands of Polish soldiers and officers surrendered to the Red Army (RKKA). On the Soviet side, 1,475 soldiers and commanders were killed. Speaking at the parade on November 7 in Red Square, Voroshilov said:
“Units of the Red Army relatively easily accomplished their task. Within a few days Western Ukraine and Western Belarus were entirely cleared of the forces of the former landowning Poland.”
▪️On 22 October elections were held to the People’s Assemblies of Western Ukraine and Western Belarus, after which deputies proclaimed Soviet authority in the former Polish territories. On 1–2 November 1939 Western Ukraine and Western Belarus were officially incorporated into the USSR.