[Corporal Aston]

If you happen to not have enough fuel to move a ship from once point of the universe to another, if you are recieving an attack, you have the following options:

• Transfer the necessary fuel/Servicemen from a colony
  
• Attack a nearby planet in order to avoid the incoming attack

Otherwise, your fleet will be immobilised if you happen to lack servicemen. But your fleet will be able to attack if it has the necessary servicemen even if it doesn't have enough fuel.