Each crossing was a potential point of ambush where a party had to slow down, concentrate, and manage its livestock through a difficult physical obstacle.

The mountain crossing — the route over or around the Big Horn Mountains — created a bottleneck that no trail engineer could eliminate.

The mountains themselves were several thousand feet higher than the surrounding plains, and the passes through them were limited in number, defined by geography in ways that were predictable to anyone who knew the country.

Predictable terrain is vulnerable terrain.

The overall length of the trail — roughly 400 miles from Fort Laramie to Virginia City — meant that even if the trail had been completely safe for the first 300 miles, the final 100 could cancel everything.