America was supplying Britain with tanks and munitions before they entered the war by putting them on cruise ships to make them look innocent. Britain later intercepted a telegram known as the Zimmerman letter, intended for mexico saying if America goes to war then Mexico should attack America and in return they will get their land back. Britain intercepted this and showed America, who later through a vote in Congress declared war on Germany. America was fresh in the war so they weren't tired from fighting, and had a lot of resources. America was able to help push Germany's lines back in the western front.