The United States was assigned ownership of the Northwest Territory (today the states of Ohio, Indiana, Illinois, Wisconsin, and part Minnesota) in the 1783 Treaty of Paris. By 1800, it had already settled much of the territory and created new states and territories. But that land could not be fully exploited because the Mississippi River system was ultimately controlled by the Spanish, and then by the French. Neither significantly colonized their North American holdings, save for French Canada. The Spanish interest was precious metals. The French interest was expensive furs. They hadn’t sent settlers into the area and therefore hadn’t cleared land and farmed it.

The English colonized the land and after independence, so did the United States. But their territories lacked one thing: New Orleans. And without controlling New Orleans, the Mississippi’s gateway to the ocean, farmers would not have access to world markets and would simply subsist on what they grew.

The Crescent City: The Key to Power

New Orleans was the key to North America. Sea-going vessels could not go very far up the Mississippi. The flat-bottom barges that brought the wealth of the Midwest down the Mississippi could not venture out to sea. New Orleans developed at the point where ships and barges could each safely meet. The barges exchanged cargo with the ships, which then carried it to Europe.

Of course, to get to this point, the plain between the Rockies and Appalachians had to be settled and farmed. This westward expansion achieved two things. The first was an enormous increase in economic power. The second was strategic depth.

In 1803, France was engaged in the Napoleonic Wars for domination of Europe, and Napoleon was not very interested in the Louisiana Territory. For the Americans, and particularly for President Thomas Jefferson, it was an obsession. The US bought the territory for $3 million dollars, which even in today’s dollar was an absurd amount—about $230 million. That price included the entire Mississippi River and New Orleans.

The defense of New Orleans became a central interest of the United States. During the War of 1812, when the British destroyed Washington, they also attacked New Orleans. Future President Andrew Jackson defeated the British there and kept control of New Orleans and the Midwest. Jackson remained properly obsessed with New Orleans. It was the key to American power and prosperity. It was also still in danger.

The US-Mexican border was only about 200 miles away from New Orleans. In order to defend it, the Mexicans had to be pushed back. This was not a trivial fear. The United States had a small standing army, spread through a large territory. The Mexicans had a larger army, and if they massed a force, they might be able to take New Orleans and strangle the United States.

In the classic paradox of American strategy, the desire to defend New Orleans triggered an attack on Mexico in two parts.