President Donald Trump will meet NATO allies in Brussels next week, who will expect the U.S. leader to again go on the attack about insufficient spending on defense by other countries.

Trump has time and again criticized member countries of the alliance that have failed to hit a target, agreed in 2014, to spend 2 percent of their gross domestic product (GDP) on military forces.

The bond between Western countries is already under stress following June’s G7 summit in Canada, when Trump criticized the leaders of Germany and Canada over trade deals.

At that G7 meeting, Trump also claimed that NATO was “much too costly for the U.S.” and has since written to NATO leaders urging them to accelerate their outlay.

The man in the middle of the debate is NATO Secretary-General Jens Stoltenberg.

He told CNBC’s Hadley Gamble on Thursday that the alliance was moving in the right direction and that President Trump recognized that.

“The good news is that we have started to do exactly that and we have started to see a significant increase,” Stoltenberg said. ”Over the past few years we have added $87 billion from Canada and the European allies and that makes a real difference.”

Article 5 of the NATO treaty states that an attack against one ally is considered as an attack against all allies. NATO invoked Article 5 for the first time in its history after the 9/11 terrorist attacks.

However, Trump has suggested that if countries did not reach their 2 percent spending target, then the U.S. would not be willing to intervene on their behalf. That threat has been seen by many as undermining the alliance’s stability, but Stoltenberg said the United States would not benefit from any scenario in which NATO was weakened.

“Two world wars and the Cold War showed us that we are stronger together and therefore it is in the interest of the United States to maintain a strong transatlantic bond.”

One country in particular that Trump is critical of is Germany, whose military spending only accounted for 1.2 percent of GDP in 2017. The share is set to rise but the increase is slow and Berlin has only committed to 1.5 percent by 2024.

In a tweet last year Trump said that Germany owed “vast sums of money” to both NATO and the United States.

Germany is currently the second largest supplier of troops to NATO, behind the United States. Its defense minister, Ursula von der Leyen, said Tuesday that looking at bald numbers was not a refined method of judging a country’s commitment.

“You can easily spend 2 percent of GDP on defense without actually offering anything to NATO,” she said in Berlin on Tuesday, before adding that contributions to NATO were more important than what a country spent on its own military.

One of the issues likely to feature on the agenda when the meeting kicks off next Wednesday will be the position of NATO members on Ukraine.

After Russia annexed Crimea from Ukraine in 2014, a move that has not been recognized internationally, Moscow said it would not return the region. In response, the United States and European Union have imposed economic sanctions on Russian companies and individuals.

NATO has previously defended the use of economic sanctions against Russia, suggesting they could prevent Moscow from invading other countries.

Stoltenberg said Thursday that all NATO members remained resolute on the “illegal annexation” and that he was not concerned that the United States could soften its stance when Trump meet President Vladimir Putin in Helsinki on July 16.

“Russia is our neighbor. Russia is here to stay and we have to strive for a better relationship and therefore I welcome the fact that President Trump will meet President Putin,” he said.

“Dialogue with Russia is absolutely consistent with NATO policies,” Stoltenberg added.