"What exactly in the Constitution prevents states from making foreign policy?"

The current challenge in Federal court is an effort to prevent the state and the city from pursuing that case.

Such steps would effectively prevent other states from following Wisconsin's lead.

That would prevent states from imposing their own tougher rules.

The budget bill would prevent states from excluding a product outright.

To prevent states from barring the sale of birth control?

But what the industry really wants is a weak national standard that prevents states from taking a more serious approach to the terrorist threat.

The tribe brought suit to prevent the state from imposing those taxes.

The act also says the Government does not intend to prevent states from legislating in this area.

The delays prevented the state and city from receiving payments for the land.