Complex sentence: This was seen as controversial because the cities--although they were part of Poland until after the war--were at that time claimed by the Soviet Union, and the British authorities did not wish to offend the Soviets.
Split into simpler sentences: This was seen as controversial because the cities--although they were part of Poland until after the war--were at that time claimed by the Soviet Union. The British authorities did not wish to offend the Soviets.