Question

What policy reasserted the U.S. position as protector of the Western Hemisphere

Answer 1

I'd say it was the Roosevelt Corollary

Answer 2

The Monroe Doctrine which was established in 1823 essentially declared U.S. control and hegemony over the Western Hemisphere. It stated that efforts by any European nations to colonize or interfere in the affairs of North or South America would be viewed as acts of aggression which would require an American response. However, the doctrine did state that the U.S. would not interfere in existing territorial or colonial claims in the region of European powers.