True but,

1) you have to have an army the AI thinks it can't beat.

2) you can do this maybe once or twice in a row but no more. The point is to annex a few more regions that are defendable. A king probably won't retreat in such a way that your new borders will be as small as possible. Provinces deep in ennemy territory are nearly impossible to defend.

3) you'll probably end up facing a huge army when otherwise you could've fought the AI's forces piece meal. It may be what you're looking for but it sure isn't the easiest way.