Donald Trump may have found the killer weapon to punish countries at will: by imposing tariffs on their exports to the United States. And he dug it out of the dustbin of history - specifically, the Smoot-Hawley Tariff Act of 1930, the very law that helped choke global trade and usher in the Great Depression. Because nothing says "Make America Great Again" like resurrecting the policies that made America Greatly Depressed.

The president has been hunting for a new tool since February, when the Supreme Court struck down tariffs he slapped on virtually all of America's trading partners under the International Economic Emergency Powers Act (IEEPA). The court deemed the invocation of national emergencies - including a "public health crisis" from illegal drugs, trade deficits that "led to the hollowing out" of manufacturing, and Brazil jailing former president Jair Bolsonaro for plotting a coup - to be, shall we say, spurious. Policy wonks have been playing parlor games predicting what legal justification he'd deploy next.

Last Friday, Trump imposed round-robin tariffs on everybody under Section 301 of the Trade Act of 1974, arguing that trading partners unfairly undercut US companies by importing goods made with forced labor. A blatantly specious accusation, sitting uneasily alongside America's own state-sanctioned abuse of forced labor. But Section 301 doesn't give him carte blanche; it's meant to retaliate against specific "acts, policies and practices" that are "unreasonable" and "burden or restrict" US companies. A blanket charge that every country enables forced labor is unlikely to fly in court.

The truly menacing precedent came from the Monday before, when Trump foisted a 50% tariff on imports from Canada. The pretext: Canadian discrimination against American dairy, booze, and cars - which, paradoxically, came about as retaliation against Trump's original tariffs. Sly observers note Trump's threat to punish Canada for forest-fire smoke wafting into the US. Maybe the duties are meant to soften Canada for USMCA talks, or to convince Canadians to become the 51st state. Or just because "tariff" is a pretty word.

The authority invoked: Section 338 of the Smoot-Hawley Tariff Act of 1930. This statute is dangerous because it lets the president slap levies on any country under hazy arguments - specifically, if that country does anything to put the US "at a disadvantage compared with the commerce of any foreign country." Once the president decides this, he can declare new duties offsetting the burden. Until Trump, Section 338 was never used to impose tariffs; it was leverage for "most favored nation" treatment. Trump doesn't care about such niceties.

America's trading partners were elated in February when a 6-3 majority, including two Trump appointees, ruled against IEEPA use. But the court showed unbelievable deference, objecting only because tariffs are taxes, the preserve of Congress. The executive has gotten away with spurious arguments before: in 1975, Nixon invoked the Trading with the Enemy Act of 1917 to tariff a Japanese zipper company - a country we weren't at war with. Trump's lawyers cited that precedent.

Since February, Trump's minions have trawled legal statutes. He invoked Section 122 of the 1974 act for a 10% tariff to prevent balance of payments crises - a crisis that doesn't exist with floating exchange rates. His own lawyers argued Section 122 couldn't substitute for IEEPA. It had a 150-day limit. He's also used Section 301 and Section 232 of the 1962 Trade Expansion Act for steel, aluminum, lumber, semiconductors, and autos on national security grounds.

None grant the unrestrained power of Section 338. With such a sprawling mandate, Trump may be difficult to stop. Let's see what he does with it - because apparently, the only thing standing between global trade and chaos is a 1930s law that already caused one Great Depression.