Writing is an agent of conquest because it is the most efficient way to convey and to store large amounts of information.
Information is very important. Information can help fleets of ships know where to go as they head off to conquer and colonize. It can tell them what they need to know about the areas they are going to try to conquer. Information can also help to organize the fleets and the armies that are needed to conquer. Taking a step back, writing even helps to collect the tax revenues that are needed to fund these fleets and armies.
Information is power. Writing is a good way to accumulate information and to pass it from person to person. For these reasons, writing is an agent of conquest.