Whenever I heard about the Monroe Doctrine in school, it was always about how the USA put it forward to protect the Latin American republics from European interference. Never once was it mentioned that the Monroe Doctrine also effectively meant the USA could exercise a veto power on any Latin American relationship with the rest of the world. The new republics never asked for the doctrine in the first place and it was useless whenever the British navy didn’t feel like blockading France… until 1898.
After 1898, the USA could use the Monroe Doctrine to extend a condition of empire over the whole of Latin America. Rather than incorporate the lands politically and then have to deal later on with questions of citizenship and rights, as did the Roman Empire, the USA allowed for political separation to exist in legal terms, but managed to nevertheless control Latin American nations through forced treaty obligations and military interventions. This, in turn, meant that US corporations could use the puppet governments propped up by US forces and US-trained forces to create unfair economic arrangements to suck natural resources out of Latin America to make cheaper goods for US citizens. Slavery existed, just not in a jurisdiction where it was both illegal and where law enforcement would act to put an end to it.