It’ll be debated for some time whether Donald Trump’s confrontation with Ukrainian President Volodymyr Zelensky was spontaneous or a pre-planned ambush, but by the end of the 15-minute exchange Friday, Trump relished how it would all play: “This is going to be great television.”

The idea that Trump has created a reality TV presidency is well-worn, but the reaction to the shout-fest, that it signals a breakdown in U.S.-Ukrainian relations and perhaps Western alliances, is hardly trivial.

“Wow, just wow,” said CNN’s Dana Bash, after the network finished running video of the exchange, before going to the network’s chief White House correspondents and then foreign correspondents to talk about the implications.

Such reactions continued throughout the day as the story dominated cable news and social media.

Trump was supposed to meet with Zelensky today to put the finishing touches on a U.S. mineral rights deal with Ukraine,...