A close assault is an attack by infantry on a vehicle, using only grenades and/or demo charges.

A close assault happens automatically when infantry are near an enemy vehicle, where "near" is quite close -- in the same or adjacent action spots. The infantry must have grenades or demo charges. The infantry will face the tank, and you will see grenade-hurling animations. Each attack has a chance to immobilize or destroy the vehicle, and the vehicle will also be suppressed. Infantry will assault a tank this way a few times per minute.