Can't say I've seen this before. I have some Soviet squads operating behind the enemy front line, and suddenly a unit not previously under my control, or even view, appeared!? Did my boys just liberate a captured unit? (Never under-estimate the Russians in winter warfare!!)
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"Never put off till tomorrow what you can avoid doing entirely."
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Yep, if you can get your units close enough to a surrendered unit they'll revert back to your control. They don't have any weapons though and can't pick any up.