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.

although world war i began in 1914, the united states did not join the war until 1917. the additional firepower, resources, and soldiers of the u.s. to tip the balance of the war in favor of the allies. remaining neutral. when war broke out in 1914, the united states had a policy of neutrality.