Military observation.
When we drive to Billings, we drive by the Battle of Little Bighorn site (just off of I-90). Looking at the terrain as we passed by today, I thought to myself Lakota Sioux, Northern Cheyenne, and Arapaho were some bad-asses in stealth technology to sneak 1,100+ warriors uphill to attack a force of 700 well-armed cavalry troops.
Just in case you've never seen what the area looks like, there's not a lot of things to hide behind and it's not heavily forested.