A unit which is assaulted by the enemy whilst falling back must check to regroup immediately (see Regrouping below). No modifiers apply to the check in this case, and even units which would not normally be allowed to regroup are allowed to make the check – the situation is sufficiently life or death threatening for the usual regrouping restrictions to be ignored. If successful the unit is regrouped and will fight the close combat normally. If it fails to regroup the unit is destroyed.
Assaulted While Falling Back
Source: Core Rulebook 3E, p. 72
A unit assaulted while falling back must immediately attempt to regroup with no modifiers or restrictions, or be destroyed if it fails.