WASHINGTON — President Trump has decided to roll out the big cannon.

In the Rose Garden on Friday, he said he was declaring a national emergency to build a wall on the border with Mexico, using money from other federal accounts, after Congress declined to authorize sufficient funds to satisfy him in legislation that averts another government shutdown.

Mr. Trump is wielding extraordinary power to get his way. Democrats strenuously dispute that there is a national security crisis on the border that warrants using the kind of presidential authority that in the past has been used for grave matters like the Sept. 11 terrorist attacks.

Here are six takeaways from Mr. Trump’s action.

Trump will go to almost any length to appease his base

Republicans in Congress, including Senator Mitch McConnell of Kentucky, the majority leader, were fine with the spending deal that they reached with Democrats to keep the government open.

But more hard-line elements within the party scalded Mr. Trump with criticism, including the Fox host Laura Ingraham and the conservative commentator Ann Coulter. But as The New York Times reported early Friday, the president felt cornered into accepting the deal and agreed to it only with Mr. McConnell’s promise to support an emergency declaration.