The deportation of Poles living in Lithuania, Belarus and Ukraine started in late 1944 on the strength of the agreement signed by polish communist government and authorities of Lithuanian, Belorussian and Ukrainian soviet republics.
Generally it lasted till 1947.
Both sides agreed to create the lists of candidates for deportation.
Lithuania:
Registered: 383,135
Deported: 197,156
Ukraine
Registered: 854,809
Deported: 772,564
Belarus
Registered: 535,284
Deported: about 150,000
The whole process of deportation took place in extremely difficult circumstances of last month of war and first years of peace. Both sides suffered huge war damage, social life was in rebirth stage, administrational and economical systems were ineffectual. The transport, communication and food supply were subordinated to the war machine, so the deportation of that scale faced many difficulties, what caused a lot of suffering and death to the deported.
The deportations had two faces. From the one side soviet republics wanted to get rid of Poles living on their territories. That why Poles became a victims of harassment and violence.
From the other side the soviet authorities wanted to prevent from leaving rural population. It was the reason for putting pressure
on polish side to finish the deportations.
About 654,000 of Poles were left behind (cannot confirm this number right now).
The tragic situation was intensified by the atmosphere among the Poles.
From the one side they wanted to leave the areas occupied by the soviets, from the other side they didn’t want to leave their houses, property and homelands.
There may be some mistakes in numbers of deported. I will verify them later.
This post is based on a website: http://www.sciesielski.republika.pl