It looks like you're discussing the territorial expansion of the United States during the 19th century, particularly in relation to the Mexican-American War and the subsequent Treaty of Guadalupe Hidalgo, which led to the Mexican Cession.
The correct completion for your sentence would be:
"The United States gained the most because it gained a large amount of land."
The turning point you mentioned highlights the significant territorial gains made by the U.S. during this period, which intensified the debate over slavery, particularly as these new territories influenced the balance of power between pro-slavery and anti-slavery states. The Compromise of 1850 was indeed an important legislative attempt to resolve these tensions by addressing the status of territories acquired from Mexico and attempting to maintain a balance between free and slave states.