Unit Understrength

[LonginusXXI]

If a unit got depleted during an operation, would they restructure the unit or combine depleted subunits to operate at a reduced strength? (Like a legion going from ten cohorts to 8, or a cohort going from the normal six centuries to 4?)

[Robert Vermaat]

Depends on the situation I guess. We know of understrength units, but cancelling units? Maybe if a legion was almost destroyed, it would not be replenished again, but if simply understrenght it would mean they still had the staff and supply in place and replenishing would be far easier.

[dadlamassu]

It would also depend on what you mean by "unit".  I have read that in the late Republic and Civil Wars many legions were understrength and retained so for fairly long periods.  In modern military usage a full strength Legion of 6,000 troops would be a strong brigade or weak division and both would be termed "formations".  While a full strength cohort of 480 men would be about a battalion strength and so would be a "unit" as would an Auxiliary cohort or ala.  A century of 80 would be equivalent to a company and termed a "sub-unit". 

Some legions, like those of Varus (XVII, XVIII, XIX), were destroyed and never reformed.  This would suggest that if a legion suffered heavy casualties but was not wiped out (lost its Eagle?) then its cohorts were probably eventually brought up to strength or to near full strength. That said depending upon the loss rate of Centurions and other leaders the cohorts may well have been temporarily amalgamated until reinforcements and promotions were made.  

It is equally likely that some sub-units may have existed as little more than cadres to preserve the illusion of a larger force.  
This, of course, is largely conjecture and applies modern military thinking to the problem.

[Praefectusclassis]

Studies of strength reports in papyri show that most units were rarely at strength in the imperial period, with examples both below *and above* 'sollstarke'.