Question

vigorous political debate accompanied territorial expansion of the united states before the civil war because
a the texas territory was still under mexican control.
b most people in the united states opposed westward expansion.
c southern states refused to provide the troops necessary for such expansion.
d any new states admitted to the union might upset the sectional balance in congress.

Explanation:

Pre-Civil War U.S. territorial expansion sparked fierce debate because the core conflict was over whether new states would allow slavery. This directly impacted the balance of power between free and slave states in Congress; adding a free or slave state would shift voting power on critical national issues. Option A is incorrect because Texas was annexed by the U.S. in 1845, so it was not under Mexican control during most of this expansion debate. Option B is wrong because westward expansion was widely supported by many Americans (e.g., due to Manifest Destiny). Option C is false as southern states generally supported expansion to gain more slave states.

Answer:

D. any new states admitted to the Union might upset the sectional balance in Congress.