There is a story (whether apocryphal or not I don't know) about Benjamin Franklin and his invention of lightning rods. The story says that when he created the lightning rods and started putting them on buildings, so that fires would be prevented by the lightning hitting the rods and not the buildings, that Christians were chastising him, saying that Franklin was thwarting the will of God. After all, if God wanted to burn down a building, who was Franklin to prevent it?
Now I suppose this story could be dismissed as a simple misunderstanding, and we could say that lightning does not (usually) serve to carry out the will of God in burning down things, but is just a natural phenomenon that God lets happen without interference. Still, it brings up the question: can people thwart God? Is that possible? Or is it that God allows us to use our free-will, and thus however we use free-will will always be in line with what God wants? Yet common sense, and experience, would seem to indicate that what we wish to do through free will, and what God wants, doesn't always match up. God doesn't just give us anything we pray for, for example, but gives us what it is his will to give us, for our benefit, as St. John seemed to be getting at in this passage:
"And this is the confidence that we have in him, that, if we ask any thing according to his will, he heareth us: And if we know that he hear us, whatsoever we ask, we know that we have the petitions that we desired of him." (1 Jn. 5:14-15)
Is there a difference between wanting and acting? Is God willing to tell us "no, that's not for your benefit" when we ask in our heads or with our mouths through prayer, but not willing to say "no, that's not for your benefit" when we physically try to bring things about? Are not both things activity, and both things part of our free-will manifesting itself? When does God thwart and when not? But to add to this, what do we do with passages such as Numbers 22, where God does seem to stop us from acting as we wish? If God sends something meant to stop us, or tell us something, and we ignore it, are we thwarting the will of God?