America's forgotten war: the Mexican-American War (1846-48) may be America's most overlooked conflict, but it helped create the U.S. as we know it today--both geographically and culturally.

• Author: Depalma, Anthony
• Position: TIMES PAST

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You probably recognize "From the halls of Montezuma" as the opening line of the Marine Corps anthem. But you may not know what the words actually mean.

As it's used in the Marine hymn, the phrase has little to do with the Aztec empire, which Spain conquered in 1521, marking the start of its colonial rule in Mexico. Rather, it commemorates the final battle in Mexico City in 1847 of the Mexican-American War, one of the most important, yet least known or understood, conflicts in American history. Although it lasted less than two years, that war has had an enduring impact on North America, determining how the western third of the U.S. became what it is today.

Historians have long debated how America's war with Mexico, which started 165 years ago this year, should be viewed. For some, the conflict was a necessary footnote to the idea of America's Manifest Destiny, the 19th-century belief among many Americans that the U.S. was meant to stretch from ocean to ocean and become a beacon of freedom to the world.

But for others, particularly Mexicans, the war is a dark spot on the history of the continent. It's seen as an example of naked U.S. aggression against a weaker neighbor that deprived Mexico of a third of its territory and left it leery of American intentions to this day.

Given the enormous impact of the war, one might expect it to be a highlight of U.S. history. But it's not.

"Most Americans don't know that the war occurred," says Timothy Henderson, a Mexico scholar at Auburn University in Montgomery, Alabama. "They are not even aware that the southwest U.S. was once a part of Mexico."

From Triumph to Turmoil

When Mexico won its independence from Spain in 1821 after 300 years of colonial rule, it stretched from the southern border of Oregon in the Northwest to the far reaches of Central America (see map, right, and timeline, p. 20). Independence created a new country, but it was weak and poorly governed. Most of its people were illiterate, and the upper classes continually fought among themselves for power.

At the beginning of the 19th century, about 5 million people lived in all of Mexico--roughly equal to the population of the U.S. at the time. But whereas waves of immigrants would double America's population every 20 years and relentlessly push westward into new territory, Mexico's population stagnated, attracting few European settlers. Even most Mexicans didn't dare move into the distant northern territories, where the new government did little to provide essential services or protection from hostile Indians.

Mexico hoped to solidify its northern region by inviting English-speaking settlers into the sparsely populated territory of Texas. Within a few years, though, the newcomers outnumbered the natives and began agitating for independence.

A bloody conflict between Texas and Mexico earned Texas its independence in 1836. (The Battle of the Alamo, a Mexican siege of Texan forces in a San Antonio mission, was the most famous battle.) Within a decade, the new Republic of Texas agreed to be annexed to the U.S. Soon, Americans began casting their eyes even farther west, all the way to Mexican-owned California.

When James K. Polk of Tennessee was elected president in 1844, he pledged to acquire the Oregon territory from Great Britain and California from Mexico.

With Great Britain, Polk took a hard line--asserting America's legal right, based on treaties and early explorations in the region, even if it meant risking another war with that country. His gamble worked, and Britain signed treaties that allowed the Oregon territory to come fully into American hands in 1846 without a shot being fired.

Polk wanted to use the same tactic with Mexico, but he misread that country's resolve. After losing Texas, Mexico understood that the U.S. appetite for land would be greater...