The cost is unnecessary tensions with the people of this land and the neighboring city. Likely even distrust.

[no city or town wants to be 'occupied' after all. but alas, he understands that this isn't exactly a time of peace... and sometimes, creating tensions are necessary.]

However... in light of this blight, I suppose it couldn't be helped in the eyes of your leadership. I don't know if I would do the same in that position. The risks are too numerous.