The Entity had abused dollar hegemony over the years by simply printing dollars. The Entity ran huge trade deficits every year. China, Japan and all other countries had had to keep reserves of dollars if they were to purchase oil and protect their currencies. These were like never- having-to-be-repaid loans to the Entity. If other currencies could be used they would dump these reserves because the only thing preventing inflation of these dollars had been dollar hegemony, backing the dollar with oil. Iran doing business in non-dollar currencies is like a leak in a dike.
Russia, China, India and Japan are now unloading dollars carefully, so as not to cause a panic. But they are steadily unloading them. They see dollar hegemony disappearing. Naturally, Saudi Arabia sees what is happening, and is not that enamored of the dollar either. As long as they thought the Entity protected them from Iran and, of course, from the Entity itself, they went along with it. Now the Entity is impotent to enforce dollar hegemony. The dollars the Saudis take for their oil today will be worth a whole lot less tomorrow if dollar hegemony ends. They are wavering, especially after Trump scolded them for murdering Kashoggi. Naughty, naughty MBS. They know the Entity cannot protect them from Iran, and they are panicking.

The end of dollar hegemony is an existential threat to The Entity. Originally The Entity exchanged securities for these dollars, one piece of paper for another, or more likely bits of code, with the Federal Reserve. Then it spent them, mostly on the military. 

The Federal Reserve unloaded these dollar-denominated securities to whoever had faith in the dollars they could exchange them for. Nice work if you can get it, but securities are debt. The Entity is so far in debt that it pays almost a trillion dollars in debt service annually. 

 If everyone unloads dollar securities the entity will have to print more dollars, and sell more securities to buy them. Otherwise their price will crash. But what real something or other will these dollars buy given how many will be floating around? For there will be no other buyers unless the dollars can buy something real. The Securities will then be worthless. If Entity securities become worthless so will the dollar. Bye-bye Entity.

Without dollar hegemony the dollar will hyper-inflate and destroy the Entity. To restore dollar hegemony it was thought essential that Iran return to the dollar hegemony fold.

With the credibility the Entity had had while it pretended to be the United States gone, and attacking Iran impossible for any sane entity, Trump is left with only one option if he is to maintain dollar hegemony: to go insane. The only alternative to going insane is to not attack Iran, allow dollar hegemony to dissipate (as is inevitable anyway), and so end The Entity– for the debt accrued through dollar hegemony is unpayable, except in hyperinflated dollars.