I know it's merciless, but if I were king, I would probably let them starve. (For one, they basically threatened my kingdom. For two: )
If they really declear war against us, they will be starved and weakened. True, a cornered enemy is very dangerous, but a starved army doesn't really do much.
If they attack the kingdom, we could probably defeat them, and get ahold of new territories, where we could distribute the food for the poor and replace the aristocrats with our own.
Tho this is just speculation.