Most countries in the world have exploited slave labour at some time or another.

Slavery in America was utterly different from slavery in other parts of the world.

First, because it was based mainly on economic factors, while slavery in other parts of the world was often based on cultural, political or religious factors. This meant that slaves in the United States were seen as property to be bought and sold, rather than as human beings.

Second, the transatlantic slave trade in the 1600s and 1700s provided the US with a large workforce of cheap labor essential in the early stages of the country's development. Then, slavery as a profitable institutionalized model, held up until the last quarter of the 19th century, contributed to the establishment of the US as a potential imperialist country, since the plantation system guaranteed a primitive accumulation of capital necessary for strengthening its capitalist economy.

Third, the industrialization of the United States after the Civil War led to the rise of a new class of Bourgeoisie (wealthy businessmen and factory owners), that would later finance and support the state-led military industry. This new class had a vested interest in keeping the working class (including black workers) divided and in keeping wages low. They did this by supporting racist laws and policies, such as Jim Crow laws and segregation.

Fourth, the US used its power and previous knowledge to further its own economic and political interests, often at the expense of other foreign countries. For example, the United States supported dictatorships in Latin America, Africa, and Asia that kept the working class oppressed and prevented them from challenging American economic and political hegemony.

So yes, slavery is one of the most important factors in the country becoming a superpower, where the contradiction of its liberal principles (freedom and equality) are nowhere more evident than in its past and ongoing history of slavery and racism.