The United States acquired Texas by annexing it in 1845. This helped to set off the war with Mexico that began the next year.
Texas came to be annexed by the US because it had seceded from the country of Mexico. Texas had always been part of Mexico until it broke away through a rebellion in 1836. The rebellion happened because American settlers in Texas did not want to live under Mexican rule. The Americans had come to Texas at the invitation of the Mexican government but decided they did not want to remain as part of Mexico. They broke away in 1836 and asked to join the US. The US did not agree to annex Texas until 1845.
During the 1820s Mexico gained independence from Spain and allowed the immigration of settlers into its sparsely populated Texas. It so happens that the majority of immigrants were Americans whose population surpassed that of the Mexicans by several fold. What followed were attempts by the Mexican government to control this population, but these attempts led to revolutions during the 1830s. Finally in 1836 Texas declared its independence from Mexico and elected its first president, Sam Houston. However, a new feud sparked following Texas’ claim of the Rio Grande border which Mexico claimed control over. At that time, Texas wanted to be annexed by the United States but there were two major hurdles. First, the US would only annex Texas as a slave state but this would create an imbalance as stipulated by the union’s constitution with regard to the number of slave and non-slave states. Secondly, Mexico had threatened war if the US went ahead with the annexation. It was not until eight years after Texas gained independence from Mexico that President John Taylor succeeded at annexing Texas through a Treaty of Annexation.
The United States acquired Texas after The Republic of Texas decided to be annexed into the United States. (Annexed means to be added as a state). After Santa Anna let Americans go into Texas, he demanded that they follow his rules, which they did not like. So, the United States went to war with Mexico over the Texas territory. After the war, Texas was not added to the United States. Instead, Texas became its own country, The Republic of Texas, for ten years. After ten years, the Texans decided that they wanted to become a state instead of being a country.