Final answer:
The expansionist movement in the United States during the 1840s, driven by the belief in manifest destiny, justified the acquisition of land as inevitable and justified. This led to the addition of Texas and Oregon to the Union and the acquisition of California through the war with Mexico. The era of expansionism also caused tensions with Native Americans and fueled the slavery debate.
Step-by-step explanation:
The expansionist movement in the United States gained tremendous momentum in the 1840s, with the belief in manifest destiny driving the desire for land acquisition. Manifest destiny justified the expansion by viewing it as inevitable, just, and divinely ordained. This led to the addition of Texas and Oregon to the Union, as well as the acquisition of California and other territories through the war with Mexico. The era of expansionism also resulted in tensions with Native Americans and fueled the debate over slavery.