On September 17th, 1939 the USSR had joined in the invasion of Poland. Soviet losses are estimated at between 8,000 and 10,000 (the Polish High Command had ordered to retreat in front of the Red Army). Eastern Poland or the regions now called WESTERN UKRAINE and WESTERN BELORUSSIA were integrated into these Soviet Republics. A communist society and economy was enforced. The captive Polish soldiers were divided in ordinary ranks and officers; the officers, many of them of noble or bourgeois background, were summarily executed and buried in mass graves such as at KATYN, an act Soviet authorities tried to conceal for years to come. The area gained hat a combined population of 14 million, of them about 5 million ethnic Poles. The Soviets immediately implemented a policy of deportation, to which 1.7 million fell victim, for the most part ethnic Poles. In addition, another hundreds of thousands were killed in a systematic de-Polonization of the USSR's recent acquisitions. This policy targetted especially the traditional elite, owners of estates, bureaucrats, teachers etc.