The Reason

Aero took a deep breath, close his eyes for a second, exhale and then open it back up and there is a smile on his face

He could guess Eric plans and his intention.

Eric probably did not use his main bulk of the army and he might even have planned to take a chunk of Savillia kingdom territory when Savillia is attacking Karak

That is a possibility worth entertaining. Savillia is not as big as Veva. Half of Veva army is probably only one third of Savillia army

The only reason why United Kingdom of Veva had never been able to decisively destroyed Savillia kingdom is because of the alliance between the Three kingdoms.

But right now, in this turbulent situation, everything might change. With almost two thirds of Savillia army in Karak right now, they have change the battlefront.

But because of that, their defense in their kingdom is low right now, vulnerable even. The rulers of Savillia kingdom had taken a big risk and had a big payout.