Final answer:
The response of Roosevelt to the London Conference indicated the United States' move from isolationism towards proactive involvement in world affairs, focusing on aiding Britain against European dictators, advocating for the United Nations, and envisioning a leading post-war role for the US.
Step-by-step explanation:
Roosevelt's response to the London Conference signalled a transition in the United States' role in global affairs, indicating a move away from isolationist policies towards a more active engagement in international politics. The failure of the conference to resolve international currency problems partly due to non-US support forecasted Roosevelt's shift in focus.
This shift became evident as he provided aid to Britain against the rise of dictators in Europe. Roosevelt's eventual leaning towards a "Europe First" strategy before America's entry into World War II signified a recognition of the global threat posed by Germany, and set the stage for the US to prioritize defeating Germany over Japan. During Roosevelt's administration, American attitudes towards international organizations changed significantly, seen in the strong public support for the United Nations by 1945.
Despite initial isolationist inclinations, Roosevelt foresaw the importance of the United States playing a pivotal role in post-war security, advocating for an informal agreement between the major powers, and later supporting the idea of a formal global organization. His vision of a postwar world free of empires and promotion of free trade augured the rise of the United States as a preeminent world power.