Former President Donald Trump is swinging back after President Joe Biden called his comments regarding NATO “dumb.”

“Could somebody please inform our uninformable President that NATO has to pay their bills!” Trump said on his Truth Social media platform.

Biden, on Tuesday, said that a weekend speech by the 45th President, in which Trump said he would encourage Russian aggression against NATO member states that aren’t meeting his standards for defense spending, was the first instance in history of a U.S. President bowing to a Russian dictator. He went on to defend the alliance formed in the wake of WWII and used a check on Soviet expansion through the cold war.

“For God’s sake it’s dumb, it’s shameful, it’s dangerous. It’s un-American. When America gives its word it means something, so when we make a commitment, we keep it. And NATO is a sacred commitment,” Biden said.

Trump, during a Saturday evening rally in South Carolina, said that during his administration he’d informed a NATO nation’s leader that he would allow the Russian military to do whatever they wanted to European countries that weren’t paying into the defense organization as much as he thought they should.

“One of the presidents of a big country stood up, said, ‘Well, sir, if we don’t pay and we’re attacked by Russia, will you protect us?’ I said, ‘You didn’t pay, you’re delinquent.’ He said, ‘Yes, let’s say that happened.’ No, I would not protect you. In fact, I would encourage them to do whatever the hell they want. You gotta pay. You gotta pay your bills,” Trump said over the weekend.

The former president doubled down on those remarks on Wednesday, insisting that NATO member states need to have skin in the game if they want the protection that’s supposed to come with joining the treaty organization. The U.S. is already footing the bill to defend Ukraine from Russian invasion, Trump said.

“They are right now paying a small fraction of what we are for the Disaster in Ukraine which, if we had a real President, would never have happened. There is probably a $150 Billion difference. They should equalize with the U.S. – FAST! The European Nations, when combined, have approximately the same size Economy as we do. They have the money. PAY UP,” he wrote.

According to NATO Ambassador Julianne Smith, spending on defense by treaty members has increased steadily for the last nine years in response to Russian President Vladimir Putin’s unlawful annexation of Crimea from Ukraine in 2014.

“Europe and Canada have stepped up and will continue to do so, especially in light of Putin’s dangerous attempts to threaten our security,” she wrote on the social media platform formerly known as Twitter.