In 1943 , during the course of World War II , the subject of Poland 's eastern borders was re-opened , and they were discussed at the Tehran Conference . Winston Churchill argued in favour of the 1920 Curzon Line rather than the Treaty of Riga 's borders , and an agreement among the Allies to that effect was reached at the Yalta Conference in 1945 . The Western Allies , despite having alliance treaties with Poland and despite Polish contribution , also left Poland within the Soviet sphere of influence . This became known as the Western betrayal
Yalta Conference
Yalta Conference
Polish–Soviet War