When European Commission President Jean-Claude Juncker sat down with President Trump for trade talks Wednesday, the atmosphere was tense. Trump had already levied harsh taxes on European imports and was threatening further tariffs on cars from the Continent, while the European Union had retaliated with tariffs on U.S. goods like Harley-Davidson motorcycles and Kentucky bourbon.

But when the two men emerged from their private trade talks, it was all smiles. "We agreed today to work toward zero tariffs, zero non-tariff barriers, and zero subsidies on non-auto industrial goods," Trump said, while Juncker added, "We have identified a number of areas on which to work together."

So how did the European Commission president turn Trump's combative trade mood into a congenial Rose Garden announcement? By playing Trump at his own game, The Wall Street Journal reported Thursday:

[O]n Wednesday, the American president was "charming, well-briefed," and "made an effort" to reach a deal with his European counterpart, the senior EU official said.

Mr. Juncker grabbed the opportunity to argue that both sides need to refrain from further punitive tariffs or they would foolishly harm themselves.

"If you want to be stupid," he told Mr. Trump, "I can be stupid, as well."

Backing up his points, Mr. Juncker flipped through more than a dozen colorful cue cards with simplified explainers, the senior EU official said. Each card had at most three figures about a specific topic, such as trade in cars or standards for medical devices. [The Wall Street Journal]

"We knew this wasn't an academic seminar," the EU official told the Journal. "It had to be very simple." Get more of the inside scoop of the Trump-Juncker talks at The Wall Street Journal. Kimberly Alters