What did Donald Trump do today?

He got confused about what side of oil prices Saudi Arabia was on.

Trump continued to do damage control today over his full-throated defense of the Saudi government. Most of it centered on the relatively low oil prices of the moment, which prompted Trump to say, "Thank you to Saudi Arabia" in a tweet this morning.

Besides promoting the idea that America's loyalty can be bought with cheap gasoline, there are a few problems with giving Saudi Arabia "thanks" for low prices.

First, the Saudi royal family—whose government is funded almost entirely by oil revenues—desperately wants and has steadfastly pursued higher oil prices through reduced supply. It allowed a blip of higher production recently to guard against disturbances to the market from embargoes against Iran, but was already throttling back production to maximize its own revenues when Trump continued his public relations campaign on behalf of the Saudi royal family.

Also, prices have fallen because of decreased demand. Here Trump can, in a way, take some credit: oil markets are slumping because of fears that his trade war will hurt manufacturing worldwide. Idle factories do not consume energy.

Finally, the fuel that goes into Americans' cars comes much more from domestic sources (the U.S. produced 76% of the oil it uses in 2015) and from Canada (which contributes 40% of the imported oil consumed by the U.S.) than from Saudi Arabia. 

Trump, who has steadfastly refused to put his business interests in a blind trust, is personally financially beholden to the Saudis.

Why does this matter?

• Presidents don't have to be the economic geniuses that Donald Trump thinks he is, but they do have to understand how supply and demand work.

• It's bad if we can't tell how much of foreign policy is being driven by a president's personal financial interests.