To understand some of the long-term effects of this tussle with Mexico, we have to understand the background of the war and the debate over Texas. 

The year was 1836 and Texans were high-fiving each other and shouting "Remember the Alamo!" as they celebrated their independence from Mexico. Mexico was looking the other way with its fingers in its ears pretending the whole thing never happened. 

America, on the other hand, was debating the idea of annexing Texas to make it a part of the Union. Texas was all for it in 1836. The only hitch in the deal was that Texas had adopted a new state constitution that legalized slavery and banned free Blacks. That wasn't going to fly with the North.

Not willing to throw off the delicate balance of free and slave states and launch America into a Civil War, America passed on Texas. We were getting really good at dancing around the whole slavery issue again and again and again. 

Strangely, avoidance wasn't solving the issue.

More and more Americans flooded into Texas over the next nine years, and by 1845, the population of Americans in the territory had more than tripled. More Americans meant more and more settlers eager for adoption into the United States. When America finally did give in to the desire for Texas, it paired it with the acquisition of Oregon to balance out the question of slavery.

All of a sudden Mexico was like "What the what? Texas is ours." 

And...war.

The war ended with American victory and a treaty that increased the nation's size by more than half a million square miles. Plus, the Oregon Territory was thrown in there, too, adding another quarter million square miles, creating a transcontinental nation-state. 

Another plus? Developments like the railroad, the telegraph, and the postal system were under way, fueling theories of Anglo-Saxon supremacy that fused with national pride to produce Manifest Destiny, the conviction that white Americans were divinely ordained to dominate the continent, from sea to shining sea.

Score, Manifest Destiny—and President James K. Polk's expansionistic campaign promises—was going well. 

Well, sort of well.

The issues that emerged with the acquisition of western lands, particularly in the Southwest, reawakened the sleeping giant of American politics: the controversy between North and South over slavery that most U.S. political leaders had spent a lifetime attempting to suppress. 

Though President Polk and Manifest Destiny embodied the spirit, adventure, and triumphalism of the period, they also paved the way for the splintering of the Union in 1861.

A narrowly elected president. A controversial war. Sharp divisions over America's foreign and domestic policy and its future path. 

Sound familiar? If not, you might want to pause here and pick up a newspaper or two.

Regardless of how you feel about the current state of affairs in American politics, warfare, and foreign relations, elements of our national history can only enhance your perspective and broaden your understanding of the country's position and reputation in the world.

• How did we come to be the way we are?
• When and how did we assume the national borders that are now taken for granted on the colorful maps adorning almost every American classroom?

Study of the Mexican-American War and the expansionist policies of mid-19th-century United States can help answer these questions. Iraq and Vietnam weren't the first controversial wars in national history. The notion that everyone in the country once zealously supported the chief executive, regardless of his party or his agenda, is a myth.

Not every square inch of American territory was gained by bloodshed, and many powerful historical figures thought that the land we did acquire was actually insufficient. If they had their way, our country would look very different today. The United States would include parts—or all—of Mexico, Nicaragua, the rest of Central America, Cuba, and a few other places, to boot. 

So, what drove those ardent expansionists and why did they ultimately fail? 'Cause we'll tell you that most Americans did actually favor aggressive territorial acquisition.

Also keep in mind that in history, nothing can be taken for granted, and hindsight's always 20/20. Never assume that our country always looked the way it does today on a map, or—as in the ideology of Manifest Destiny—that we were destined to assume our present form. 

Newsflash: History doesn't neatly unfold according to a series of predetermined—or deterministic—prophecies. No matter how Americans have rationalized their land conquests, or whether they were fighting Mexicans, Native Americans, or others, these were messy, bloody, protracted battles on all sides.

The outcome was neither obvious nor easy. To trivialize the process of westward expansion or imperialism as an inevitability is to oversimplify history. Even if we now know the end of this particular story—that we did in fact conquer the continent, "from sea to shining sea"—it is the how and the why that really counts. The but is also pretty important. In other words, what was the cost of westward expansion?

You could think about that question in terms of the lives lost—white settlers on the frontier, American soldiers, Mexican soldiers, Mexican citizens, Native Americans—or the tribal cultures that were decimated by disease and bloodshed along the way. Or the price of the many land treaties in cold hard cash, the dramatic alterations to the landscape and the resultant changes in the environment, or the impact on national goals, values, and ideas. 

The Civil War must be part of that answer, too. Even if most 19th-century Americans agreed on the virtues of expanding the national boundaries westward, they disagreed violently over whether slavery should expand into the west as well.