Answer:
The Monroe doctrine stated that the U.S. should use military force to prevent any form of intervention from an European power in the western hemisphere.
The Roosevelt corollary was different in that it allowed European powers to intervene as long as the intervention was considered justified, but not to invade.
This difference became clear during the Venezuelan crisis of 1902-1903, when Germany, Britain, and Italy imposed a naval blockade on Venezuela because the Venezuela president refused to pay for the damages suffered by European citizens during the recent civil war.
At first, Theodore Roosevelt allowed the blockade to continue because he believed that it was justified, but when Germany threatened to invade Venezuela, he intervened sending a fleet under Admiral George Dewey.
