In 1804, after years of harsh rule, the slaves of Haiti overthrew their French oppressors, and Haiti became the first black republic. The United States looked unfavorably upon this fledgling republic, for fear that it may lead to domestic unrest amongst its own slaves. Haiti and the United States had a strained relationship for many years, and the constant political turmoil in Haiti led to more disdain from the United States.
In 1915, an angry Haitian mob attacked their current President, and in a scene of unimaginable violence, they literally tore him to shreds. This event, along with the history of deposed presidents, led the United States to deem Haiti unfit to be self-governed. The Marine Corps landed and seized control of the country with relatively little resistance.
Keeping in line with the current progressivism and "good neighbor policy", the United States genuinely tried to rebuild the country. However, there may have been more motives than one might suspect.
America was becoming concerned with the increasing German presence in Haiti. German businessmen had attained many positions of power in the country. The United States, with one eye already focused on the situation in Europe, did not want to worry about a possible German naval base in the Caribbean.
Although America did contribute much to the health and education of the country, the racism of the Marines offset these gains. The Marines were hand-selected, and many came from the South. Those in the south treated the Haitians as if they were inferior to whites. Jim Crow Laws and other racial barriers were established in Haiti. Many of the government posts were taken from black Haitians and given to White or mixed-race Haitians. This racism established a rigid caste system that favored the minority population.
This intervention happened during a time when the idea of social Darwinism was gaining popularity, and the African-American population in the United States wanted Haiti to prove that Africans are equally capable of self-government. The Haitians saw American blacks as inferior, because they did not rise up in rebellion against their oppressors.
The American intervention, whether its intentions were true or self-serving, was often seen as Imperialism and racism. After the withdrawal in 1934, the country, now disarmed, was ready for a strong centralized government that could rule by force, not the desired intention.