A problem started to brew on the border between the United States and Mexico, along Rio Grande. In June 1845, President Polk sent people to help resolve the dispute on the border. John Slidell came up with an offer, but Mexican officials wouldn't speak to him. In March 1846, General Taylor led his troops to the Rio Grande, and they were told to back down by Mexican forces, but refused. A mini battle broke out and several people were killed. Two days later after President Polk's messages the Congress declared a war on Congress.
At the start of the war with Mexico, the United States had better weapons, and better apparatuses, but they were overwhelmed and were not at all prepared. A plethora of Americans encouraged the war, however on the other side, the Whigs thought the war was needless. The war continued, even though Americans were arguing about the ethics. President Polk sent General Stephen Kearny to invade New Mexico. On August 18, 1846 he took control of New Mexico.
In 1846 an estimate of 500 Americans resided in California compared to the 12,000 Californios who already resided there.. With the thought of the manifest destiny in their mind, a group of American settlers decided to capture Sonoma. In the Bear Flag Revolt, Americans wanted California to become an independent nation. The Americans captured a few important Californios. However, the Bear Flag was quickly brought down by US naval forces.
Eventually General Taylor got hold of a better army and began to ride into Mexico. When Antonio Lopez de Santa Anna heard about him, he quickly grouped his troops, and went after Taylor and his army. They both clashed at Buena Vista in February 1847. The Mexican Amry retreated, and the General Taylor got very popular, and wouldn't listen to President Polk. So, he sent General Winfield Scott to Mexico. He defeated the Mexicans, then Santa Anna fled the country.