Why was Mexico concerned about Texas joining the US?

* Texas was a large and strategically important territory. It was about the same size as France, and it had a long border with Mexico. If Texas joined the US, it would give the US a much stronger position in North America.

* Texas was a slave state. Mexico had abolished slavery in 1829, and it was worried that Texas would bring slavery back to the region.

* Texas was a hotbed of American settlers. Many Americans had moved to Texas in the 1820s and 1830s, and they were often at odds with the Mexican government. Mexico feared that these settlers would eventually try to overthrow the Mexican government and annex Texas.

In an attempt to prevent Texas from joining the US, Mexico sent troops into Texas in 1836. This led to the Texas Revolution, which resulted in Texas gaining its independence from Mexico. However, Mexico never officially recognized Texas' independence, and it continued to claim Texas as its own territory.