As President Trump declared a national emergency on Friday to bypass Congress and build his long-promised wall, he again painted a portrait of a lawless, chaotic border and cited arguments about the effectiveness of the kind of barrier he has in mind that were not rooted in facts.

Illegal border crossings have been declining for decades. While families are overwhelming an immigration system devised to handle single men, a border wall would not prevent them from seeking asylum, which is legal. Research does not show that immigrants commit more crimes than native-born Americans. And a wall would do little to prevent drugs and human trafficking at the border, as official ports of entry are the main route into the United States for both.

Cumulatively, Mr. Trump’s unsupported or misleading statements undercut his rationale for declaring an emergency, a step that is widely viewed as testing both constitutional and political norms and is sure to draw legal challenges.

What Was Said

“In El Paso they have close to 2,000 murders right on the other side of the wall. And they had 23 murders. It’s a lot of murders. But it’s not close to 2,000 murders right on the other side of the wall in Mexico.”

This is misleading.

Mr. Trump is correct that there were 23 murders in El Paso last year, compared with more than 1,200 murders in Ciudad Juárez, Mexico. But the disparity predates the existence of border barriers separating the two towns.