Which was the last major land acquisition the us made prior to the beginning of the civil war

Mexican Cession. Mexican Cession lands were captured in the Mexican–American War in 1846–48, and ceded by Mexico in the Treaty of Guadalupe Hidalgo, where Mexico agreed to the present Mexico–United States border except for the later Gadsden Purchase.

answer; /// i do believe that its napolean's conquest of spain;

yes.

explanation:

because it let the japanese know who had the upper hand in the war.