The vehicle came off the road at Laxford Bridge
Two gunners have escaped with minor injuries after their vehicle toppled off a bridge and fell 30ft into a river.
The members of 29 Commando, Royal Artillery, were travelling in a Drops transporter to Cape Wrath in the far north of Scotland.
Their vehicle came off the A838 Lairg-Durness Road at Laxford Bridge.
The Army said the pair had cuts and bruises. Highland Council said the road would be closed for some time.
The transporter was carrying a digger.
Major structural damage has been caused to the bridge, closing it for the foreseeable future, the local authority said.
People travelling to and from Rhiconnich, Kinlochbervie and Durness faced a detours of 60 to 100 miles via Altnaharra and Lairg.
A resident of Kinlochbervie, a village of about 450 people, said the community had effectively been cut off.
Pupils living on the south side of the bridge were unable to attend the local high school.
The gunners were travelling north on Wednesday to take part in Exercise Joint Warrior - Europe's largest military - which will run for three weeks from Monday.
The Army in Scotland said military police were working with Northern Constabulary on the investigation into the cause of the accident.
Campbell Stewart, Highland Council's area roads and community works manager, said the authority was aware closing the bridge for repairs would would be a "major inconvenience" for its regular users.
He added: "However, the bridge has been severely damaged and must remain closed for safety reasons.
"We are currently assessing the structural damage involved and as yet it is too early to say how long the bridge will be closed and how much it will cost to repair it."