The U.S.A was wanting to settle the entire North America and called this Manifest Destiny and believed that God wants them to expand.  They believed that it was destined by God for them. It was done under President Polk's administration. If you were a American at this time you would be going out and settling west because of many reasons like more land to farm and settle in. 
The Mexican-American war was a war between America during 1846-1848.If you were a American at this time you would be helping America fight and win the war to help fulfil the manifest density goals. 
The Civil war broke out between the South and North over many issues like state rights but the most important issue it broke over was slavery. If you were a American at this time you were either fighting for the North or South and it was a time of conflict within your own country. 
The recontruction followed the Civil War and was a era of rebuilding after the Civil War. Its major purpose was right after the secssion of the states in the South and bringing back the South into full agreement with the Union. If you were a southerner at this time you would've lost all your slaves and be forced to take care of your own farmland without slavery and many southern farmers crops failed.