The Monroe Doctrine was A United States policy opposing European interference in America. This policy viewed any European effort to gain control back over the newly independent countries in America as "the manifestation of an unfriendly disposition toward the United States." It was issued in 1823 under the presidency of James Monroe, in a moment that most Spain and Portugal colonies in AMerica were fighting for their independence or trying to build independent nations.
The original aim of this policy was to prevent the New World to become a battle field for the Old World powers, so the United States could exert its own influence undisturbed.
The Venezuela crisis was a perfect scenario to apply the Monroe Doctrine, because European powers were using military force to press an American country to pay its debt. This could have been seen as "an unfriendly disposition toward the United States". What Roosevelt did was add the Roosevelt corollary to the Doctrine, which asserted the right of the United States to intervene in Latin America in cases of "flagrant and chronic wrongdoing by a Latin American Nation" to preempt intervention by European creditors.
This changed the meaning of the Doctrine , which went from a policy of defending the American countries' independence to a policy that allowed the U.S. military interference in Latin America when it failed to pay European or U.S debtors from then on.