President Donald Trump stunned U.S. allies Wednesday during what was supposed to be a photo op before a bilateral breakfast meeting with NATO leaders. Instead, Trump blistered NATO Secretary-General Jens Stoltenberg over NATO defense spending and Germany's close relationship with Russia.

All 29 NATO members are meeting in Brussels Wednesday and Thursday for the organization's annual summit.

What happened?

Trump's tirade was initiated by a reporter who asked Trump which NATO countries he believes need to spend more money on their own defense budgets. NATO defense spending has long been a major issue for Trump, who takes issue with European allies not paying the pledged minimum on their own defense.

According to a 2006 NATO agreement, each country pledged to spend at least 2 percent of its gross domestic product on defense. The shortfall has led the U.S. to bridge the gap in defense of NATO allies.

"Take a look at the chart," Trump instructed the reporter, explaining the U.S. is owed a mass of money for decades of delinquent allied payments.

Stoltenberg responded cordially, noting Trump's leadership for bringing the issue of defense spending to the forefront of NATO's agenda. Stoltenberg stressed that progress has been made and pledged further progress in the near future.

What did Trump say about Germany?

When Stoltenberg mentioned Trump's upcoming meeting with Russian President Vladimir Putin, Trump tore into Stoltenberg.

"Well, I have to say I think it's very sad when Germany makes a massive oil and gas deal with Russia when you're supposed to be guarding against Russia, and Germany goes out and pays billions and billions of dollars a year to Russia," Trump said.

"So, we're protecting Germany, we're protecting France, we're protecting all of these countries. And then numerous of the countries go out and make a pipeline deal with Russia where they're paying billions of dollars into the coffers of Russia. So, we are supposed to protect you against Russia," he explained.

"And I think that's very inappropriate. The former chancellor of Germany is the head of the pipeline company that's supplying the gas. Ultimately, Germany will have almost 70 percent of their country controlled by Russia with natural gas," Trump continued. "So you tell me: Is that appropriate?"

"It should have never been allowed to have happened. But Germany is totally controlled by Russia," Trump declared. "I think it's a very bad thing for NATO."

What else?

Trump later returned to NATO spending, explaining Germany is paying just 1 percent of its GDP, while the U.S. is paying 4.2 percent of a much larger GDP. Trump said the reality is "unfair" to American taxpayers.

"And I think these countries have to step it up not over a 10-year period. They have to step it up immediately," Trump said. He later returned to ripping Germany, explaining the country is "captive to Russia."

What gas deal was Trump referring to?

NPR explains:

Trump was referring to a 2015 agreement between Russian gas giant Gazprom and Germany to build a pipeline under the Baltic Sea. Despite German Chancellor Angela Merkel's tough talk over Russia's annexation of Crimea and sponsorship of separatist rebels in eastern Ukraine, she has defended the $11 billion pipeline deal to supply her country's energy shortfall.

Trump is scheduled to meet with German Chancellor Angela Merkel later Wednesday, where he is expected to address Germany's NATO spending and its pipeline with Russia. He is also expected to meet with Russian President Vladimir Putin in Helsinki on Monday.

How did leaders respond to Trump's tirade?

Stoltenberg said:

NATO Sec.-Gen. Stoltenberg on his meeting with President Trump this morning: "Of course President Trump has a very direct language and message on defence spending, but fundamentally we all agree. All of us agree that we need fairer burder sharing in the alliance." — NBC News (@NBCNews) July 11, 2018

While Merkel commented: