Final answer:
The most concerning problem with Texas joining the United States was how it upset the balance of slave states and free states created by the Missouri Compromise.
Step-by-step explanation:
The subject of this question is History.
The most concerning problem with Texas joining the United States was how it upset the balance of slave states and free states created by the Missouri Compromise.
The Missouri Compromise, enacted in 1820, sought to maintain a balance between slave states and free states in the United States. According to this agreement, Missouri entered as a slave state while Maine entered as a free state, and a line of latitude (36°30') was established, with slavery prohibited to the north of it. Texas, on the other hand, entered the Union as a slave state in 1845, disrupting the delicate balance established by the Missouri Compromise.
Learn more about Texas joining the United States and the Missouri Compromise balance