Civilization IV/Military

Strength
A unit's strength is the indicator on how well it performs in battle. It is based on the base strength of the unit, and is modified by terrain, abilities, and HP of the unit. The strength will be fixed once a unit enters battle, but will drop if injured.

To see the combat odds, select your unit, hold Alt and hover your mouse over the enemy unit.

First strikes
A first strike is a chance to inflict damage without retaliation. While combat checks are performed as normal, units do not take damage while performing first strikes.

Collateral damage
The collateral damage ability from siege units inflicts a light amount of damage across multiple units. This is used to clear out a large number of enemies within the same tile, and is useful if you need to weaken a large number of units before you engage with your combat units.

Withdrawal
A unit with the withdrawal ability has a chance of disengaging from combat. This is performed at the last moment; if successful, the unit will retreat with only minimal health.