If God wants something done, can you, by your exercise of Free Will, prevent him from doing it?
And otherwise...it depends.
Think of the warnings of Mary that without prayer, a great error would emanate from Russia to spread throughout the world.
Hmmm, Communist much?
On the other hand, you have Esther, who was told if she failed, God would raise up a deliverer for the Jews by other means.
I don't think we're always in a position to know what would have happened.
Cheers!