The territory across the Mississippi was largely unexplored and attracted frontiersmen who mapped and colonized much of the area. James Nolan, who left New Orleans in 1792 to horse trade with the Spanish and Indians in what is now Texas, was typical of these early pioneers. After the Louisiana Purchase in 1803, the area around New Orleans and territories to the west of the Mississippi became part of the new US, doubling its size. The US annexed the southern coastal area and negotiated the purchase of Spanish Florida. Meanwhile increasing numbers of pioneers began crossing the continent and encroaching on Indian tribal lands, where they were often attacked. The relationship between the Native Americans and the migrants was one of trade and war, with trading centres established in Pawnee, Comanche and Apache lands. There were many fortresses around the Gulf of Mexico in Louisiana and around the Kansas and Missouri rivers.