What was the main reason why the United States did not allow Texas to become part of their country?

The issue behind Texas annexation was slavery.  According to the Missouri Compromise of 1820, Texas should have been able to come in as a slave state since it was south of Missouri; however, since there was no free state to come in with it, this would have led to an imbalance in the Senate during a time when the United States government was having trouble keeping slaveholders and abolitionists happy.  Texas would have to wait for nine years before achieving statehood.  

There was also the issue of provoking a war with Mexico.  

Explanation:

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Northerners opposed slavery and annexing Texas would upset the even number of those whom apposed and those whom agreed. They also wanted to avoid war with Mexico.

Explanation:

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