President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE demanded on Wednesday that NATO members immediately increase their defense spending to meet a 10-year goal years ahead of the alliance's stated deadline.

"What good is NATO if Germany is paying Russia billions of dollars for gas and energy? Why are there only 5 out of 29 countries that have met their commitment?" Trump tweeted shortly after he scorned allies for not spending more on their own defense and relying too heavily on the U.S. for protection.

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"The U.S. is paying for Europe’s protection, then loses billions on Trade," he added. "Must pay 2% of GDP IMMEDIATELY, not by 2025."

NATO members agreed in 2014 to move toward spending at least 2 percent of their gross domestic product (GDP) on defense by 2024.

Trump has repeatedly railed against NATO allies' slow efforts to meet that goal. As he attended the NATO summit in Brussels on Wednesday, he demanded that alliance members spend at least 4 percent of their GDP on defense — a goal that not even the United States currently meets.

The U.S. spent 3.6 percent of its GDP on defense last year.

A number of NATO members have increased their defense spending, and several are set to meet the 2024 goal.

Trump took credit on Wednesday for the increase in defense spending by several alliance members. He acknowledged that his predecessors had broached the issue of defense funding with NATO allies in the past, but said that they “never did anything about it because I don’t think they understood it or they just didn’t want to get involved.”

He also said that the hikes are "not nearly enough."

"Many countries are not paying what they should. And, frankly, many countries owe us a tremendous amount of money for many years back, where they're delinquent, as far as I'm concerned, because the United States has had to pay for them," Trump said.

"So if you go back 10 or 20 years, you'll just add it all up. It's massive amounts of money is owed,” he continued.

Contrary to Trump’s suggestion that allies owe the U.S. for money spent on NATO’s collective defense, the spending goals agreed upon by member states have to do with their individual defense budgets rather than the alliance as a whole.

Those efforts have been unsatisfactory to Trump, however, who has for months pushed allies to increase spending and has complained that the U.S. is unfairly burdened with higher costs for little gain.

"You know, we're protecting Germany, we're protecting France. We're protecting everybody. And yet we're paying a lot of money to protect," Trump said.

The U.S. president’s latest comments are reflective of his skepticism of the transatlantic alliances that were forged or strengthened in response to World War II and became a linchpin of global security during the Cold War. The remarks also reflect Trump’s notion that the U.S. is essentially being ripped off by its closest allies.

Trump’s comments on Wednesday came on his first day in what will be a series of meetings throughout Europe. When he leaves Brussels, he will travel to London for a long-awaited visit, before stopping at one of his golf courses in Scotland.

He is then expected to meet with Russian President Vladimir Putin in Helsinki for a highly anticipated summit.

The flurry of meetings has stirred some concern that Trump could be gearing for a repeat of the Group of Seven summit in Canada in June, when he arrived late, left early and delivered a blistering rebuke of trade with U.S. allies, particularly Canada, before jetting off to Singapore for a meeting with North Korean leader Kim Jong Un.

While Trump agreed with other NATO leaders on a plan to strengthen the alliance’s defenses against an increasingly aggressive Russia, the president did not hold back on criticizing European allies on other fronts.

He claimed, for example, that Germany is "captive to Russia" because of a gas pipeline deal between the two countries.

"If you look at it, Germany is a captive of Russia," Trump said during his meeting with NATO Secretary-General Jens Stoltenberg. "They got rid of their coal plants. They got rid of their nuclear. They're getting so much of the oil and gas from Russia. I think it's something that NATO has to look at. I think it's very inappropriate."

Stoltenberg attempted to downplay Trump's concerns and urged unity, while acknowledging that a group as large as NATO is bound to have disagreements.

"The strength of NATO is that despite these differences, we have always been able to unite around our core task, to protect and defend each other, because we understand that we are stronger together than apart," he said.