Combat endurance is the time that a military system or unit can remain in combat before having to withdraw due to depleted resources. The definition is not precise; for example the combat endurance of an aircraft, without qualification, is usually the time the aircraft can remain at an altitude suitable for combat, but in a particular theatre of operations it is the time it can remain in the area of combat. During the Battle of Britain, for example, the combat endurance of German fighters was the time they could remain over Britain, i.e., their inherent (endurance)less the time to travel from their base to Britain, and the time to return—about 15 minutes.