Why was England an ally to the US during world war 2?

It was really the other way round. Germany invaded Poland on 1st September 1939. On 3rd September 1939 Britain and France declared war on Germany. In 1940, France was invaded by Germany and British troops were evacuated back to the UK. Due to strong public opinion, the US refused to become involved in the war but sent food and supplies to help Britain. After the attack on Pearl Harbour by Japanese forces on 7th December 1941, Germany declared war on the US and attacked US shipping in the Atlantic. Churchill and Roosevelt met and agreed to pool resources and that the Armed Forces of both countries would work together to defeat both Germany and Japan which they eventually achieved in 1945.