When the Second World War began, the Allied forces wanted to stop the expansion of Nazi Germany in Europe. Germany had been annexing neighboring lands, such as the Sudetenland in Czechoslovakia, for several years prior to the beginning of outright hostilities. When they outright invaded Poland on September 1, 1939, Great Britain and France declared war on Germany in order to halt these territorial advances.
Unfortunately for France, they were soon defeated and occupied themselves. The USSR initially remained neutral, occupying some of Poland themselves, but the German invasion of Russia in June of 1941 pulled that nation into the conflict. In December of that year, the United States joined the allied forces as well.
By this time, the goals of the Allies had shifted somewhat. It had become clear that Nazi Germany was too aggressive of a nation simply to be pushed back to its own original borders. The main Allied powers of Great Britain, the United States, and the USSR instead pushed for the complete and unconditional surrender of Germany. They knew that a peaceful and stable Europe would not be impossible as long as a Nazi state continued to exist.
As the war progressed, it became clear that the Allies needed to work together to reclaim and liberate all of Germany's territorial expansions tp completely defeat the country. Only by doing this did they believe that a peaceful post-war Europe could be established.