Your suggestion is perfectly reasonable. My main objection was to the idea that the Empire launched an all-out attack on basically a knee-jerk reaction.

I mentioned Sun Tzu not because he was a particular visionary, but because he was a collector of common wisdom and conventions. My point was that people were smart enough to understand basic strategy since antiquity.

Here's my idea: the Emperor is hosting a conference with other powerful leaders to arrange a possible truce or temporary alliance until the nature of these invaders can be decided. Fantasy characters can be ambassadors, guards, advisers, spies, or otherwise persons privy to the meeting. That should work for the most of the characters, right?