Impact on the US
The Texas annexation had both its positive and negative impacts on the United States. First the negatives. Because Texas clearly favored slavery, it threatened the balance in congress between free and slave states, a very hot topic at the time. To throw congress out of balace would mean almost certainly slavery would forever be a part of the US. Not only that but Texas had recently beaten Mexico to declare its independence, and many were worried this would spark tensions between the US and Mexico. Not to mention the still disputed boundaries of the new border. In the end, James K. Polk annexed Texas, thinking the good outweighed the bad.8 Unfortunately, this did lead to a war with Mexico. The very heavily disputed border was the cause. A US military unit advanced to the mouth of the Rio Grande where Texas claimed its border was. Mexico, thinking it was further back, crossed the Rio Grande, and this was seen as an invasion. Congress declared war on Mexico.
There were positives to come out of this war though. The US would win and acquire a huge plot of land through the Treaty of Guadalupe Hidalgo in 1848, as well as the Gadsden purchase in 1854. This land would eventually become Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming from Guadalupe Hidalgo and extra parts of Arizona and New Mexico from the Gadsden Purchase.7 This would lead to the reduction of Mexico by over half its land. While the territories won provided more land for the US, it brought about very heated debates about who should be a slave state and who should be free, and eventually the Civil War broke out.