When President Trump warned Germany about their dependence on Russian natural gas, European leaders laughed. Today, they’re learning just how right he was, as they pay a steep price for not heeding the warning.

In his 2018 speech at the United Nations, Mr. Trump said, “Germany will become totally dependent on Russian energy if it does not immediately change course.”

He urged them to buy their supply from their allies in North America, adding, “Here in the Western Hemisphere, we are committed to maintaining our independence from the encroachment of expansionist foreign powers,” meaning Russia.

The Washington Post reported at the time that, “[I]t probably won’t be the script that will be remembered by diplomats but, rather, world leaders’ laughter, caught on camera and shared in viral videos.”

Germany’s foreign minister, Heiko Maas, they said, “could be seen smirking alongside his colleagues,” but Russia’s invasion of Ukraine has wiped that smirk off his face.

On Sunday, Germany’s economy minister, Robert Habeck, warned of gas supplies getting “really tight in winter.”  The day Mr. Trump warned about is looming on the horizon.

Yet because Europeans didn’t like him — and thought they didn’t need Americans anymore — they chose to cuddle up to the Russian bear.

Germany sucked up Russia’s natural gas, becoming Moscow’s largest market in Europe. So their government didn’t dare to threaten cancellation of the Nord Stream 2 pipeline linking their nations to forestall the invasion.

That threat might have given the Kremlin pause, but Berlin only ended the project after the invasion began, under international pressure, too late to preserve the peace.

Chancellor Angela Merkel, according to the Post during Mr. Trump’s presidency, made clear that Nord Stream 2 “was not driven by business interests alone but also by political motivations.”

The Social Democrats, “her key coalition partners” as the paper described them, “have a long record of favoring stronger ties to Russia” at the expense of America.

The European Union as a whole became dependent on Russia more each year, until it accounted for about 40 percent of its natural gas, and 55 percent of Germany’s.

The Wall Street Journal reports that now, this reliance “has raised the specter of a potential fuel shortage if Europe goes into winter with less-than-full stowages.

“It has also raised prices, putting additional pressure on economies that are already struggling with high inflation and rising borrowing costs and face the prospect of a recession.”

Mr. Trump also warned that Germany had become “a captive to Russia” before a 2018 NATO summit in Brussels. Again, he faced derision, but his judgment proved sound.

“Germany will have almost 70 percent of their country controlled by Russia with natural gas,” he told the NATO secretary general, Jens Stoltenberg, in objection to Nord Stream 2. “You tell me, is that appropriate?”

Germany thought so then, but now — along with Italy, Austria, France, Slovakia and others — they are learning what happens when you give the very nation NATO was set up to oppose the power to cut off your energy.

Again, Mr. Trump had it right, warning Mr. Stoltenberg, “We have to talk about the billions and billions of dollars that’s being paid to the country we’re supposed to be protecting you against.”

Europeans had a good laugh on that one too, until the war came. Last week, the chief executive of Russia’s Gazprom, Alexei Miller, announced that they would cut natural gas supplies to Germany in half, no doubt with a smirk of his own.

“Our product, our rules,” he told the St. Petersburg International Economic Forum, leaving Germany scrambling to reactivate coal plants they’d planned to mothball in pursuit of greenhouse gas reductions.

For their government’s shortsightedness, everyday Germans have been forced to pay more, conserve and find other sources of energy as winter approaches, which — as World War II taught them — favors the Russians.

Europeans may look down their noses at Americans in general and Mr. Trump in particular, but the war in Ukraine has taught them a hard lesson.