George Friedman, of Stratfor, described the settling of Texas, the independence of Texas and the later Mexican American war as being part of the same process:

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.

First, Jackson asked Sam Houston to organize American settlers in the northeastern section of Mexico and foment an uprising designed to, at the very least, block Mexican access to the region… and at best, create an independent country, the Republic of Texas. This was accomplished in 1836 when Sam Houston defeated Mexican forces under Santa Anna at the Battle of San Jacinto, near today’s Houston.

The second stage took place in 1846 when the United States, now more militarily capable, conducted a broad assault on Mexico, including amphibious operations that led to the capture of Mexico City.

The Mexican-American War achieved three things from the American point of view. First, it crippled Mexican military capabilities for over a century. Second, it created a barrier between Mexico and the United States. After the war, there was a string of deserts and mountains south of the new border that made any possible counter-move by Mexico difficult. Finally, the US took control of all of northwestern Mexican territory, which included present-day California. This made it possible to secure the Louisiana Territory against any potential threat from the west and anchor the United States on the Pacific. It created the framework for the contemporary continental United States.
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