In 1939, 17 days after the German invasion, the Soviet Union also invaded Poland. There was no formal declaration of war by either side. The Polish government in exile established in London maintained contacts with its representatives in occupied Poland, the Polish Underground State. In 1943 following the revelations about the Katyn massacre and Polish government insistence on investigation, the Soviet Union broke off the diplomatic relations with the London Polish government, intending to establish a competing structure of power.