Call to Arms

A Call to Arms is issued by the Head of a House to assemble the military forces of their vassals to attack an opposing colony. Vassals may reject a Call to Arms from their master, but doing so is usually dishonorable.

Codex
"When a ruler or leader needs to participate in a military venture, they issue a call to arms to their vassals in a particular star system. Each vassal is obliged to show up at the location with the combined fleets of their house and their vassal houses.

This is the most important obligation that a vassal has towards their master, and breaking it can justify military retaliation (at a later date) by the master.

Ships are expensive to supply, and a house is not able to supply it's full fleet if the call to arms happens at a location that is far away from its home colony. As a result, the fleet that a leader can muster with a call to arms can be significantly different in strength in different systems. You can mouse over or click on a system to see how much power a leader or ruler can project in that system, i.e. the base strength of their fleet if all their vassals answered a call to arms."