Yeah... for some time I have been thinking about Sembia's role in all this. I think that strife between the Dalesmen, the drow, and the elves would be to their benefit. A long term political gamble that might be to their benefit would be the following...
Support in what ways they can the peace treaty, with the intention of creating a split between the elves and the Dalesmen. The elves have long been a thorn in Sembia's side, with their goals of logging and other such acitivies.
After some time has passed, with the treaty between the drow and the Dalesmen in place and the elves are no longer allied with the Dalesmen, Sembia will then encite a war between the Dalesmen and the drow. This may be done by any number of means, either by assisting the drow, or by making it look like some attacks were their doing, etc... shouldn't be a difficult task either way.
Once the war has begun once again, the Dalesmen will find themselves without their elven allies, and that's when the Sembians offer their assistance. An army happens to already be prepared near their borders... and they'll be glad to come in with overwhelming numbers and stop the drow. Of course, at the end of this the Sembians might wish several dales as their own... and if the Dalesmen wish help with those pesky elves, they'll be glad to assist.
Yep, pure evil.