Manifest Destiny was this idea that Americans should expand and move and own all the land between Alatanic Ocean and Pacific Ocean. Some long terms effects were, Native Americans kept getting shoved around. Mexicans were in the way, so Mexican - American War (War for Texas, California, New Mexico, Nevada, and Utah.
Manifest Destiny also pushed for USA to go into Canada. Hence 54 - 40 or fight with President Polk. Read more in your text book for answers. Enjoy.