I cannot explain better:

«The specter of protectionism is rising.  It is always a dangerous and foolish policy, but it is especially dangerous at a time of economic crisis, when it threatens to damage the world economy.  Protectionism’s peculiar premise is that national prosperity is increased when government grants monopoly power to domestic producers.  As centuries of economic reasoning, historical experience, and empirical studies have repeatedly shown, that premise is dead wrong.  Protectionism creates poverty, not prosperity. Protectionism doesn’t even “protect” domestic jobs or industries; it destroys them, by harming export industries and industries that rely on imports to make their goods.  Raising the local prices of steel by “protecting” local steel companies just raises the cost of producing cars and the many other goods made with steel.  Protectionism is a fool’s game».