In the beginning of the 1900's, many Latin American countries had established large amounts of debt to European countries. Countries like Germany, Italy, and Great Britain began to identify ways for their loans to be repaid. In 1902, these countries teamed up and sent gunboats into Venezuela. These gunboats blocked ports that Venezuela used to trade with the rest of the world. These European countries were sending a message to other Latin American countries. Venezuela had defaulted on its debts to foreign bondholders and they didn't want the other Latin American countries to do the same.
When the news of these block-aids reached American, many Americans began to worry. America had begun to develop some dominance in the Western Hemisphere. The actions of those European countries could undermine that dominance. President Theodore Roosevelt's Annual message to Congress on December 6, 1904 changed how the United States would now react in world affairs. Roosevelt added a corollary to the Monroe Doctrine. The proposition said that the United States would not interfere in the affairs of any country that have business with Latin American nations so long as they conduct their affairs in a polite fashion.
If not, the United States would intervene and act as international policemen. His message announced that European nations would not be allowed to use force to collect debts owed to them by Latin American countries. Roosevelt believed that the biggest problem he faced was not European intervention but the need to establish governments in Latin America that would maintain "order within their boundaries and behave with a just regard for their obligations toward outsiders." Roosevelt reminds them of the unchanged status of America's unique interests and position in the Western Hemisphere, complete with direct promises of America's new commitment to policing even the paying of obligations. Roosevelt feared that foreign nations, especially Germany, might intervene forcibly to collect their debts. To keep other countries out and ensure financial solitude, Roosevelt felt something was to be done to stop any possible force by any European country. So Roosevelt tied his policy to the Monroe Doctrine, that he would win the public's acceptance.
The corollary was also accepted by many of the Dominicans who then asked the United States for help. The United States took over the collections and used the money to pay Santo Domingo's foreign debts. Roosevelt and later presidents cited the corollary to justify intervention in the Dominican Republic, Cuba, Nicaragua, Mexico, and Haiti. Until Roosevelt's message to Congress, the United States was favorable to revolutions that propelled former colonies into a democratic age. They were against all intervention in any part of the world. Roosevelt opposed revolutions as a cause for disorder and favored strong governments.
Roosevelt declared U. S. intervention to be a rightful means of establishing and maintaining order. Before Roosevelt's amendment, the Monroe Doctrine was there to prevent European intervention in the Western Hemisphere, this now justified American intervention throughout the Western Hemisphere. Over the long term, the corollary had little to do with relations between the Western Hemisphere and Europe, but it did serve as justification for U. S.
intervention in many of the Latin American countries.  Walter LaF eber, Inevitable Revolutions: The United States in Central America (1983); Dexter Perkins, A History of the Monroe Doctrine (1955).  The Roosevelt Corollary to the Monroe Doctrine (web).