Answer: A. The vote in Congress to annex Texas
Explanation: In 1836, Texas gained independence from Mexico and expressed its desire to join the United States. However, the U.S. Congress had concerns about expanding slavery and therefore delayed the annexation of Texas for several years. This delay was due to the ongoing debate and tensions surrounding the issue of slavery.
Finally, in 1845, the U.S. Congress approved the annexation of Texas. This decision was seen by Mexico as an act of aggression because they still considered Texas as part of their territory. Mexico strongly opposed the annexation and viewed it as a violation of their sovereignty.
As a result, tensions escalated between the United States and Mexico, leading to the Mexican American War in 1846. The war lasted for about two years and ended with the Treaty of Guadalupe Hidalgo in 1848, in which Mexico ceded a large amount of territory to the United States, including California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado, Wyoming, Kansas, and Oklahoma.