In America, the earliest colonists who arrived weren't skilled at farming. They rarely, if at all, practised crop rotation: the process by which you plant different types of crops so that you didn't exhaust the nutrients in the soil.

The settlers didn't do this. As a result, they would use up the land until it could no longer provide produce. They always needed and coveted virgin soil to farm. Soon much of the arable land in the Thirteen Colonies was exhausted and settled, while the population grew exponentially. The only way they could find more land was to expand into native territory west of the Appalachians.

Early on, they would negotiate treaties with various indigenous tribes (who vastly outnumbered them initially) to obtain land in exchange for European goods, tools, payments, political/commercial alliances or arms. But while the natives saw such treaties as an agreement to share custody of the land and its resources, the white settlers saw the treaties as formally ceding the land to them as private property. Diseases from the Old World, such as smallpox, decimated the indigenous populations, and soon, the growing colonial populations were better able to defend themselves with the aid of troops from Europe.

Disagreements between whites and natives on the frontier would often flare up into hostilities thoughout the 1600s and 1700s, exacerbated by the European powers (mostly England and France) who allied themselves with rival tribes in their quest for more resources and to further imperial ambitions on the continent.

After the Seven Years War aka French and Indian War, the 1763 peace treaty established a Proclamation Line that imposed a boundary of white settlement roughly along the Appalachians. Colonists were not supposed to settle west of it, as this was "Indian Territory". But the need for unspoiled land for agriculture was insatiable and American settlers largely ignored the limits. They were incensed that Britain would try to limit their westward expansion. To put salt in the wound, Britain added a large swath of this land (S. Ontario and Ohio Valley) to Quebec -- a now-British colony of French Catholics, who the xenophobic New England colonists loathed as long-time enemies in an era of rampant sectarianism. This, and tensions over taxation and representation in the Thirteen Colonies, fueled the grievances that would spark a revolution.

The American Revolution forced the indigenous tribes to choose sides, with most backing Britain in the hopes of stemming the tide of American westward expansion. The war's end didn't resolve the conflicts over expansion in the Ohio Valley and frontier wars and raids between whites and natives occurred regularly up to the War of 1812, when the natives, under the charismatic leadership of Tecumseh, sought to halt American expansion and create a native homeland by supporting Britain in the war.

With Tecumseh's death in 1813 and Britain's later betrayal of the native cause during the 1814 peace negotiations at Ghent, any hope for a separate native territory was lost. The U.S. continued the systematic removal of indigenous peoples from fertile lands to reservations throughout the 19th century. Tribes were pressured or coerced into treaties to cede land, often for unfair compensation or promises of aid re: education, healthcare, economic support, etc. that were later reneged on.

As the U.S. expanded west of the Mississippi and plains after the Civil War, they waged wars of pacification against any indigenous groups who didn't come to terms with the U.S. gov't and resisted resettlement.