War broke out against Mexico in May 1846, and the United States Army numbered a mere 8,000, but soon 60,000 volunteers joined. The American Navy dominated the sea. Morale was on the American side. The war was a rout.
President Polk directed the war from Washington, D.C. He sent a 4-prong attack into the Mexican heartland. John Fremont and Stephen Kearny were sent to control the coveted lands of California and New Mexico. Fremont led a group of Californians to declare independence—the "Bear Flag Republic" was not taken seriously, but Fremont and his followers marched to Monterey to capture the Mexican Presidio. By 1847, California was secure.
Kearny led his troops into Santa Fe in 1846 causing the governor of New Mexico to flee. The city was captured without casualties.
The attack on Mexico proper was left to two other commanders. Zachary Taylor crossed the Rio Grande with his troops and fought Santa Anna's troops successfully on his advance toward the heart of Mexico. Winfield Scott’s troops marched to the capital, Mexico City; all that remained was negotiating the terms of peace.
At home, the Whigs of the north complained about the war, questioning Polk's methods as misleading and unconstitutional. Abolitionists feared that southerners would use newly acquired lands to expand slavery. Antiwar sentiment emerged in New England. Writer Henry David Thoreau was sentenced to prison for refusing to pay taxes he knew would fund the war.
The Mexican-American War was formally concluded by The Treaty of Guadalupe-Hidalgo. The United States received the disputed Texan territory, as well as New Mexico territory and California. The Mexican government was paid $15 million. The United States Army won a grand victory.
Source: The Mexican-American War
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