On 28 April 1947 Operation Vistula was launched and lasted until 30 July. It was a forced resettlement of post-war Poland's Ukrainian minority (especially Boykos and Lemkos) from the south-eastern parts of Poland to the recovered territories. It was carried out by the Soviet installed communist authorities. The stated goal of the operation was to suppress the Ukrainian Insurgent Army (UIA/UPA), which had been committing crimes on Poles in those south-eastern territories since 1943. The number of expelled is estimated at 140,000 or 150,000 persons.
Antoni Zakrzewski: The Operation 'Vistula'