The logistic support has already built various temporary bridges in the UK
A temporary footbridge spanning the River Derwent in Workington will be built by the army.
Royal Engineers from Tidworth in Wiltshire will build the bridge reconnecting the north and south sides of the flood-hit town.
The rising waters destroyed the Northside Bridge and left the Calva Bridge unstable and dangerous.
The 200-strong round-the-clock military effort is being co-ordinated by the Brigade for North West England.
Work to install a new prefabricated steel bridge has begun and foundations are expected to be in place by Sunday.
'Get people moving'
The bridge then needs to be assembled and dropped into place.
It is expected to be open to the public by 5 December.
Engineers are still exploring options on road connections over the river, but at this stage the immediate priority is to link the two communities.
The bridge will be positioned 300m upstream (east) of the Calva Bridge and access will be from the park on the south side.
Brigadier Bill Aldridge, Commander 42 (North West) Brigade, said: "We are very pleased to have been asked to help the civilian authorities and assist the local community in Workington with constructing this bridge across the River Derwent.
"The Army is very much part of society and part of the fabric of Cumbria where my brigade is based and we are determined to do what we can, where appropriate, and in partnership with the civilian agencies, to assist in the recovery from these devastating floods."
Councillor Jim Buchanan, leader of Cumbria County Council, said: "Our highways engineers have been pulling out all the stops and working with the army to make this happen as soon as possible.
"It's vital that we get people moving and able to cross the river."