One of the most significant events that shows Roosevelt's movement away from isolationism is the Lend-Lease Act. This act, passed in 1941, allowed for the US to trade/aid members of the Allied powers (such as Great Britain, France, and China) with military technology and assistance. Even though the US is directly helping the Allied powers, they did not send any members of the US military to help.
Ultimately, this allowed the US to stay "neutral" for the time being. This all changes when Pearl Harbor is bombed by the Japanese, as the US declares war the next day.