At first glance, it would seem that Mexico and the United States should have been good neighbors in the nineteenth century—they were both relatively young countries that had rebelled against a mother nation to establish republics. As they grew, they both had to deal with states with populations that believed that their needs were not being met by the federal government.

However, one of the biggest differences was that the U.S. population was mostly European in origin. They had come to “America” as political or economic refugees and banded together to drive out the British, French, Spanish, and Native American cultures. Feeling morally and religiously superior, they were swept along by the concept of Manifest Destiny. In the United States, the economy, especially industry and transportation, was growing quickly.

Mexico, on the other hand, under Spanish rule, had established a caste system and provincial isolationism. Its immigrant population was largely Spanish and in Mexico primarily as an occupational force. Mexico’s northern frontier was huge, politically adrift, and difficult to govern. The communication, transportation, social, and economic ties among the provinces were limited. Mexico’s new fragile republic, founded in 1821, was wracked by political revolutions for its first 25 years as different factions tried to wrest control of the government. This instability damaged economic and industrial development, and the treasury department was often in a state of near-bankruptcy.

In the spring of 1846, two armies gathered along the Río Grande near Matamoros. Zachary Taylor’s Army of Occupation arrived on the north side of the river to protect the new U.S. state of Texas and began to build a fort. General Mariano Arista’s Army of the North watched the U.S. troops warily from Matamoros on the south side of the Río Grande. Mexican cavalry patrols splashed across the river and skirmished with U.S. dragoons. By the time President James K. Polk received official word of the first U.S. casualties, several skirmishes and two major battles had been fought in the thick chaparral along the Río Grande. Claiming that Mexico had “invaded our territory and shed American blood upon the American soil,” President Polk stirred Congress into declaring war on May 13, 1846.

The Battles of Palo Alto and Resaca de la Palma in May 1846 were startling victories for Taylor’s outnumbered force and shocking losses for the proud Mexican army. In the choking heat, dust, and smoke, the U.S. army had demonstrated its cohesiveness as a unit, its fighting spirit, and the deadly effectiveness of its artillery. The Mexican retreat to Matamoros and southward to Monterrey was the start of a slow, inexorable advance by Taylor and his army into northern Mexico.

After occupying Matamoros and Camargo and breaking in new volunteer soldiers, Taylor marched southward to Monterrey, where he defeated the Mexicans again in a brutal, four-day fight in September In the city’s streets and among demolished homes.