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:

Some policymakers believed that it was the responsibility of the United States government to eradicate fascism and radicalism that was flourishing in many countries of the east. They argued that if the United States government doesn't intervene and stop the growth of fascism, it would be troublesome for the United States in the future to counter the fascist powers later.