America expanded westward from the 13 colonies because the growing nation needed more space for an increasing population and more natural resources. The 1803 purchase of the Louisiana Territory vastly increased the space available for pioneers and settlers, and the United States offered incentives for entrepreneurs to move west and establish their own homesteads. Manifest Destiny was the belief that America was destined to expand west to fill the continent.
The increasing population and shift westward quickly caused a host of problems for the young nation. First, the lands settlers were moving into had been occupied by Native American tribes for centuries, and in many cases these Native residents were forcibly moved further west or offered a pittance for their lands. Atrocities such as the Trail of Tears, a forced march of displaced Native Americans, resulted in the deaths of thousands so that valuable lands could be taken over by settlers.
The expansion into the open, lawless west brought additional problems that ultimately tore the country apart. In the former colonies, clear lines were drawn that separated slave states from free states, and both sides wanted to expand their influence over the new territories. Disagreements over which new states would enter the Union as free or slave states ultimately led to violence, failed attempts at compromise and the Civil War.