Why did the soviet union join the allies?

Russia joined the Allies, because Germany invaded its territory

Explanation:

The Allies were the countries officially opposed to the Axis Powers during the Second World War. At the beginning of the war, on September 1, 1939, the coalition confronting Germany consisted of France, Poland and the United Kingdom of Great Britain and Northern Ireland, to which they joined shortly after, some Commonwealth countries (Canada, Australia, New Zealand, the Domain of Newfoundland and the South African Union), the British Raj and the Kingdom of Nepal. In 1940, Free France, Denmark, Norway, Belgium, Luxembourg, the Netherlands, the Kingdom of Greece and the Kingdom of Yugoslavia (1941) were united. In June 1941, the so-called Operation Barbarossa led the USSR to join the Allies. The United States of America joined in December 1941, after the Japanese attack on Pearl Harbor, followed by China and some countries of South America. From 1942, the so-called Big Four, that is, the United Kingdom, the Soviet Union, the United States and China, dominated allied politics.

it's lower nubia : ) good luck

under god i have placed my chiest strentgh and the safe guard to my loyal subjects