As the new ruler of the Empire I can safely say holding your own lands with Sarleons breathing down your throat at Seven Cross is difficult, let alone launching a co-ordinated offensive against your enemies.
If you want to expand against the Sarleons without a month-long (or more) siege of Laria that means you need to take Seven Cross Keep, which is almost like a chokehold between that bridge, the pass to Sarleon, and the Laria township. It's also the place where all the enemy lords seem to regenerate at. Seven Cross Keep really lives up to its name, it's the real gateway to the Sarleon/Empire lands and an extremely important tactical location.
Unlike the Imperials who just sort of sit around and guard their homeland with those militia patrols, the Sarleons are really offensive and like to grab for any low-defenses castle they can get. King Ulric is notoriously bad for sneaking in the backwater places and taking Walven or Maras castle when I'm not looking. At least in my game.
Judging by their personality the Imperials are supposed to be like this. They keep the peace and enforce the Emperor's law, keeping the troops close to home as a display of power, and using them to improve the infrastructure of the nation.