Units adjacent to a guard unit cannot be attacked except by bombers and artillery.
How it works
If a guard unit gets deployed on the board, adjacent units in the same line will become guarded by that unit and thus cannot be attacked except by bombers or artillery. Guard is a passive ability and will automatically apply.
The position of the unit on the battlefield is important as only units adjacent to the guard unit will receive the protection.
Note that the headquarter can be guarded as well as any other of your units.
To remove the protective effect of the guard unit, you either need to destroy the guard unit itself, issue an Order (e.g. Overwhelming Force or Breakthrough), or use an effect from another card (e.g. T19 Howitzer).
How to identify Guard units
Guard units can be identified by the little corporal insignia below the unit on the battlefield.
When you look at the card, you will see the word "Guard" written beneath its image.
The story behind Guard
Guard units have a long military tradition. However, the presence of guard units on the battlefield has changed over time. Guard units often have been (elite) units with high loyalty and focus on defending either important locations or important people. In recent times, however, the responsibilities of guard units have shifted more to ceremonial responsibilities - though they remain highly skilled and well-trained.