The first Europeans to settle the Gulf Coast were primarily the French and the Spanish. The Louisiana Purchase, Adams–Onís Treaty and the Texas Revolution made the Gulf Coast a part of the United States during the first half of the 19th century. As the U.S. population continued to expand its frontiers westward, the Gulf Coast was a natural magnet in the South providing access to shipping lanes and both national and international commerce. The development of sugar and cotton production (enabled by slavery) allowed the South to prosper. By the mid 19th century the city of New Orleans, being situated as a key to commerce on the Mississippi River and in the Gulf, had become the largest U.S. city not on the Atlantic seaboard and the fourth largest in the U.S. overall.