It was in April of 1836 when the Texans stormed the mission again under the command of Sam Houston, reclaiming the Alamo as their own and gaining independence from Mexico.
Texas would remain as its own independent nation for nearly ten years. In 1845, Texas' legislature voted for annexation to the United States.
1848 brought the Mexican War which came to an end with the Treaty of Guadalupe Hidalgo. This treaty brought even more land to the United States.
The treaty brought us parts of modern-day Colorado, Arizona, New Mexico and Wyoming. as well as the all of California, Nevada and Utah. The remaining parts of Arizona and New Mexico would later be purchased from Mexico in the Gadsden Purchase of 1853.
The United States is clearly the aggressor here. Manifest destiny played a huge role in our conquering of Mexican territory.