Most Americans believed that expansion across N america was their "destiny" was expansion inevitable? What forces might have stopped it? How would american history have changed if the Mexican war had not occurred?

Hard for me to say how it would have changed, had war not happened with Mexico... But as an American one thing I know for sure... War is inevitable.
Look at our history, if we are not at war with others, we tend to be at war with ourselves.
We are just a hostile group of idiots basically.