The settlers were unfortunately attacked because the US govt violated land treaties. Newly arrived settlers didnt know that treaties were violated. What else are the Indians supposed to do? They are protecting their home. When their land was illegally taken from them and settlers moved in, the buffalo was driven away from the hunting grounds. The buffalo was a source of survival for the Lakota and all plains Indians. Food, clothing , tools.
Wow .. taking a very significant period of complex territorial expansion into a simplistic comment. The issue of treaties and the buffalo kill came at the very tail end of the Native American population decline .. it really is an incredible history to read.
The estimated population "at contact" (meaning 1492 when Columbus 'supposedly' discovered the new world and exploration of the continent was initiated) ranged from 10 million to 20 million Native Americans. By 1650 that population had declined to "under 6 million" as a result of smallpox, measles, and flue epidemics. By 1800 the population had declined to an estimated 600,000 Native Americans ... over 90% population decline primarily due to diseases (many associated with the arrival of Europeans). By 1860 .. when the 'Western Expansion" was seriously underway .. impacting the Comanche Territories (West Texas to Oklahoma) and the Northern Plains Indians .. the population had declined to 339,000 .. again .. primarily due to disease epidemics.
As far as Indian treaties .. nobody abided by the treaties .. White man or Native American. In 1860 there was a situation in which just over 300,000 Native Americans were roaming over a territory that covered what is now North Dakota, South Dakota, Montana, Wyoming, Kansas, Nebraska, Idaho, Iowa, Arizona, Nevada, Utah, Colorado, New Mexico and West Texas. That is a land area of 913 million acres with a Native American population of 300,000 made up of a dozen different tribes. A key reason that no treaties were ever abided by was that the Native American tribes generally DID NOT HAVE a single leader that spoke for all. The most fierce warriors .. the Comanches .. were made up of at least 6 different "groups". Some wanted peace .. signed a "treaty" .. but Quanah Parker .. the fiercest chief / warrior continued to fight and slaughter (butcher .. literally .. and roast alive) white settlers who thought THEY were protected by a treaty.
The biggest issue with the "Indian War" period .. was that policy was set in Washington by people who really didn't know anything about the reality on the ground on "The Western Frontier" .. and they wouldn't delegate to the men on the ground who actually worked with the Indian Chiefs and knew how to work with them. (Really interesting book is Empire of the Summer Moon .. Rise and Fall of the Comanches by S.C. Gwynne).
Have a great 4th of July!