A successful assasination attempt seems to stop the movment of at least that commanders units. I can not say for sure, but when I assasinate a lot, I seem to face smaller than expected armies, which I can often defeat in detail as Max suggested.
I gave up trying after about half a dozen failed attempts.
Though I did have a N1 mage and 10 troops take a well defended province in a amazing display of magic and fighting. He had one man left when the enemy broke and ran with overwhelming numbers.
So if the other guy zigs when you zag you can win a province.