I'd imagine that he watched what happened when the queen attacked Libertas very carefully and what the Free Cities did in their own attack in return. It's strategically sound to go for all of your enemies at the same time when you can. At the risk of repeating what you already know, I'd say he's thought the same of the Summoned since what waits on the other side of the door for you is more danger.
[ Something he knows well from being the one moving the various chess pieces around the board of war, even if his had been in defense to Edelgard making the same moves Sidwell is. Though that's an uncharitable thought: Edelgard began a war, but even her war was nothing like this. ]
How many Summoned in Thorne are familiar with dealing with a treacherous court? One even beyond what you've already experienced there, that is.
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[ Something he knows well from being the one moving the various chess pieces around the board of war, even if his had been in defense to Edelgard making the same moves Sidwell is. Though that's an uncharitable thought: Edelgard began a war, but even her war was nothing like this. ]
How many Summoned in Thorne are familiar with dealing with a treacherous court? One even beyond what you've already experienced there, that is.