The United States bought 828,000 square miles in 1803 when Thomas Jefferson agreed to the Louisiana Purchase. The U.S. had only sought to buy New Orleans, but was offered a bargain on the whole thing. Still, New Orleans was a very important part of the deal. The mouth of the Mississippi River would become the country's busiest trading port. When Americans started to move into New Orleans, they found the small town to be very foreign. The few thousand French creole residents were very different from the folks back home.
There were two main areas in which the entrenched French creoles made the incoming Americans crazy. The first was infrastructure: when the U.S. bought the Louisiana territory, New Orleans had no paved roads, no street signs, and no colleges. Much of the population was illiterate, and justice was dispensed according to the French legal code: Tregle calls the place “a colonial backwash of French and Spanish imperialism.”
The second was the permissive culture: Sunday in New Orleans means sitting at a café and going out dancing or perhaps to a horse race. In this city, black and white people mingled more freely than elsewhere in America, and even slaves had more leeway to move freely than in other cities.
All this shocked the Protestant, Puritan-minded American settlers, many of whom came from places in the South where the movement of black people was highly restricted and regulated. (Meanwhile, the native creole population was appalled by the crude Americans, who they called Kaintucks and vulgar Yankees.) The Anglo-American settlers tried to change everything from the city’s laws to the looser culture, but even as they gained power of New Orleans’ commercial life, they did not have enough political power to mold the city as they would have liked.
That changed in 1836 when the "Yankees" finally had the political power to persuade the state legislature to split New Orleans into three separate municipalities: one American and two French. Read how that worked at Atlas Obscura.