Thinking about fleet warfare in the Traveller universe again. An admiral wants to be certain they can defeat any defenses in a system they attack. A defending fleet is all fueled up and can choose to jump away if an overwhelming force jumps in. The attacking fleet takes at least a week to refuel and jump away, so unless they can fend off the attacker and secure fuel, they are stuck.
There are two fleets that attack the imperium, but are they trying to conquer worlds or just raid them? How long can they loot a system before their holds are full and they need to turn back?
Although since the war lasts several years they certainly have time to raid and retreat multiple times.