What did Donald Trump do today?

He threatened to turn the Strait of Hormuz into a protection racket.

Yesterday, Trump said the Strait of Hormuz was open to commercial shipping. This is a lie, and an obvious one: ships are abandoning all plans to transit the strait because Iran has been attacking ships that do. Cargo ships and especially oil and gas tankers are vulnerable to cheap, low-tech attacks that can be launched from within Iranian territory.

Today, Trump doubled down, saying that not only is the strait open, but that he would somehow charge a 20% surtax on the value of cargo shipped through it. This, presumably, would be in exchange for military protection of the kind he has repeatedly promised throughout the war, but never actually delivered. (There is no practical way for a navy of any size to constantly escort ships through the Persian Gulf.)

The Hormuz Strait is OPEN, and will remain OPEN, with or without Iran. We are reinstating the THE IRANIAN BLOCKADE, so named because it is only stopping Iran's ships or customers from entering or leaving. All other countries will have fair and open use of the Strait. The U.S.A. will be, from this point forward, known as "THE GUARDIAN OF THE HORMUZ STRAIT," but as such, and as a matter of FAIRNESS, will be reimbursed, at the rate of 20% on all cargo shipped, for any and all costs necessary to do the job of providing safety and security to this very volatile section of the World. The process and formation will begin immediately. Thank you for your attention to this matter! President DONALD J. TRUMP

Iran's government responded with mocking agreement, saying that Trump "is absolutely right. Whoever provides secure and safe passage of commercial vessels through the Strait of Hormuz should be compensated for this service."

Of course, this parallels Iran's own plans to charge "fees" on ships, which is a blatant violation of international law—and one they had no ability to make before Trump gave them an opportunity to prove that they actually could make good on their threats. But there are two important differences. First, Iran's threat is effective because is willing to attack or seize ships of American allies—and the vast majority of traffic that goes through the Gulf is flagged to US-friendly nations. 

Second, the fees Iran has said it will impose as part of any peace deal are much lower, about 1-3% of the cargo value. In the extremely unlikely event that Trump were able to collect a 20% tax, it would immediately cause a spike in prices, which would directly harm the American economy.

Oil prices spiked anyway today, not because Trump's threat is being taken seriously, but because of the effect of the closure Trump insists isn't happening.

It is possible that Trump is serious about reinstating the naval blockade against Iran, which is something he can actually do. If so, it would mean that Iran has had more than a month of unobstructed sales of oil at very high market prices, with Trump's permission. This has likely yielded billions of dollars in revenue for the regime.

Why does this matter?

• Making empty threats that no one believes just calls attention to your weakness. 

• The United States military isn't there to run mafia-style protection rackets. 

• There is supposed to be some difference between your side and the enemy side in a war.