In May of 1846, the United States formally declared war on the Federal Republic of Mexico, largely due to the intricate backdrop of the annexation of Texas. The tensions between the two nations had escalated after Texas gained independence from Mexico in 1836, and many in the U.S. viewed its annexation as a manifest destiny. The most pivotal moment leading to war was the incursion of Mexican troops into the disputed territory along the Rio Grande, where a skirmish broke out, leading to American casualties. President James K. Polk adeptly capitalized on rising anti-Mexican sentiments in the U.S. and pushed for war, asserting that American blood had been shed on American soil. The war was thus framed as a necessary measure to protect American citizens and uphold sovereignty over its territories.

The military conflict, which would last until 1848, involved several significant battles including those at Palo Alto and Resaca de la Palma. American forces, under the command of generals such as Zachary Taylor and Winfield Scott, swiftly advanced into Mexican territory. Polk’s administration aimed not only to solidify American claims over Texas but also sought to acquire further territories including California and New Mexico, which were vital for economic expansion and the fulfillment of the doctrine of manifest destiny. The war raised questions about slavery in new territories and set the stage for increased sectional tensions within the United States.

The conflict concluded with the signing of the Treaty of Guadalupe Hidalgo in 1848, which resulted in the U.S. acquiring vast territories including California, Nevada, Utah, and portions of Arizona, New Mexico, Colorado, and Wyoming. This territorial expansion contributed to the emerging national debate over slavery and significantly affected America's geopolitical landscape in North America. The war not only intensified American expansionist ambitions but also foreshadowed the internal conflicts that would follow in the decades to come.