(CNN) Just 72 hours removed from signals being sent out of the White House that the President would sign a bill to keep the government open that did not include the $5 billion he wants for a border wall, Donald Trump reversed course Thursday -- telling Republican congressional leaders that he would not sign the short-term continuing resolution passed by the Republican-controlled Senate on Wednesday.

"The President informed us that he will not sign the bill that came up from the Senate last evening because of his legitimate concerns for border security," outgoing Speaker Paul Ryan told reporters after he and a handful of other GOP leaders huddled with Trump at the White House.

There was some inkling that Trump was moving in this direction. "When I begrudgingly signed the Omnibus Bill, I was promised the Wall and Border Security by leadership," Trump tweeted on Thursday morning . "Would be done by end of year (NOW). It didn't happen! We foolishly fight for Border Security for other countries - but not for our beloved U.S.A. Not good!"

But the finality of the announcement -- delivered by Ryan, not Trump -- throws Washington into total chaos. Consider: a) a good chunk of the government will shut down unless Congress finds a way to fund it (and Trump signs such a bill) before midnight Friday; b) it's not clear whether many of the House Republicans who lost reelection bids will come back to DC (and do so very quickly) if they are needed to pass some sort of border wall funding measure; and c) the math in the Senate to pass a major increase in the cash allocated for the border wall is hard to see, at best, and impossible at worst.

What led Trump to change his mind? To go from an annoyed willingness to keep the government open (and avoid being blamed for the shutdown) to a refusal to do so unless more money for his border wall made it into the legislation?

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