Question

How was the Roosevelt Corollary a major change to the Monroe Doctrine?

Answer 1

For the first time the United States would police disputes between Latin American and European governments.

Answer 2

Roosevelt Corollary was part of the Monroe Doctrine articulated by the then President Theodore Roosevelt in his State of the Union address in 1904.

The corollary states that the United States would police disputes between the European countries and Latin American countries to enforce legitimate claims of the European powers, rather than having the Europeans press their claims directly.