According to the manual
Quote:Units with Assault = 0 are automatically overrun when attacked at only
the cost of movement into the defending hex.
But HQ's, which have no documented assault value, defend like normal units when assaulted and don't vacate their hex. It seems some information is missing from the explanation in the manual.
I was under the impression, based on the rule, that an HQ by itself would withdraw when attacked, but that is not the case.
Edit: After checking the OOB, it seems HQ's have an undocumented assault value of 1. The big company sized Soviet HQ's are credible defensive units.