Supply trains were raided.

Cattle were driven off.

Mail carriers were attacked.

The connection between the posts and the broader military and civilian world became unreliable.

And the civilians the road was supposed to serve?

They were not coming.

The practical impossibility of making the journey safely — wagons were attacked, people were killed, livestock was taken — meant that the Bozeman Trail had effectively ceased to function as a civilian emigrant route even as the Army was bleeding to keep it open.

The forts existed, technically.