What is the date of the Texas annexation and why is it so important?

The date of the Texas annexation was 29th December 1845. This date is important because in a few days from this date the Mexican-American War began.

Explanation:

Out of approximately 38,000 people that lived in Texas before the Mexican American War, about 30,000 were Anglos and only about 8,000 were original Mexicans.

The Anglo leaders decided to enter into a union with the United States as a slave state and hence, the United States annexed Texas towards the end of December of 1845. This annexation of Texas by the US made the Mexicans feel teased and thus, they called a war against the United States

December 29, 1845

Explanation:

The United States did not want to annex Texas because it would cause rivalry. So when Texas was annexed it was important because it started a war with Mexico. And It made Texas a part of the U. S. and no longer and independent nation.