A tax on imported goods — historically the chief revenue source of pre-modern states and the main lever of trade policy — and the site where a government's need for money, the politics of protecting home industries, and the theory of free trade collide.
In 1930 Congress raised taxes on more than twenty thousand imported goods, and the world trade system buckled. Within three years, the value of global trade had fallen by roughly two-thirds. Whether the Smoot-Hawley Tariff caused the Great Depression or merely worsened it is still debated. But the tariff sits exactly at the intersection of three things governments can never stop arguing about: where state money comes from, who gets protected from foreign rivals, and whether open borders for goods are good for ordinary people.