why did some policy makers feel the us had to switch from isolationism
to a more expanding role around the world?

Some of the United States policymakers believed that they should switch from isolationism to a more expanding role around the world because, without the United States’ intervention, many fascist powers around the world were taking deeper roots and that was believed would turn out to be dangerous for the United States in the future.

Explanation:

travels along an axon. this triggers a wave of electrical activity that passes from the cell body along the length of the axon to the synapse