What was the purpose of the Monroe Doctrine? Did the Monroe Doctrine impact the United States involvement in the affairs of other parts of the world?

Initially, northerners did not want missouri to be included in the union because 1.) maine would not be allowed to enter the union as a free state 2.) missouri wanted to enter the union as a free state 3.) missouri wanted to enter the union as a slave state 4.) maine would not be allowed to enter the union as a slave state