1846-1860

1. 领土扩充背景：
 One journalist, named John O'sullivan, wrote that the United States had a"Manifest Destiny "to spread out across North America.” 
Manifest Destiny meant that the United States was clearly meant by God to spread out to the Pacific Ocean。
During the 1844 presidential election, James Polk ran for president against Henry Clay. Polk wanted to expand the United States. He wanted texas to become part of the country. He also thought the United States should claim the Oregon Territory
2. 美墨战争
美国非法移民进入德州 ->支持德州独立 ->德州加入美国 -> 美墨战争 -> <瓜达卢佩.伊达尔戈条约》包括整个加州，内华达，犹他州，亚利桑那，新墨西哥，科罗拉多和怀俄明。
3. 西进原因：土地空间，宗教，黄金（Sutter-owner, Brannan-spreader）。
4.俄勒冈车道Oregon Trail 始于俄勒冈的独立城
5. 铁路：随着铁路的到达，原住民无处可退了，被迫进入保留区。