President Donald Trump, upon returning home from a World War I memorial event in Paris, unloaded on the US's European allies and appeared to threaten to pull out of NATO.

French President Emmanuel Macron was critical of Trump's leadership and politics during the Paris trip and floated the idea of forming a European army that would in part defend the continent from the US.

Trump called the idea "very insulting" and returned to his old talking points challenging NATO.

Trump said he told US allies in Paris that US protectorship of European countries amid trade deficits could not continue.

President Donald Trump on Monday unloaded on the US's European allies, and appeared to threaten to pull out of NATO, upon returning home from a World War I memorial event in Paris, where French President Emmanuel Macron openly rebuked Trump's political philosophy in a speech on Sunday.

Trump returned to his old talking points — that the US is treated unfairly within NATO while maintaining trade deficits with those countries — as Macron talked up the idea of a European army that would in part serve to protect the continent from the US.

Macron floated the idea before Trump's trip, and Trump described it as "very insulting."

"Just returned from France where much was accomplished in my meetings with World Leaders," Trump tweeted on Monday morning.

"Never easy bringing up the fact that the U.S. must be treated fairly, which it hasn't, on both Military and Trade," he continued. "We pay for LARGE portions of other countries military protection, hundreds of billions of dollars, for the great privilege of losing hundreds of billions of dollars with these same countries on trade."

Trump typically condemns any kind of trade deficit with any country, though the metric usually indicates the US has a strong economy that can afford to buy more from a given country than that country can buy from the US.

Read more: Here's how NATO's budget actually works

"I told them that this situation cannot continue," Trump said of the military and trade relationships with some of the US's closest allies. He described the situation as "ridiculously unfair."

The US by far spends the most in NATO, both on its own defense budget and on programs to increase the readiness and capabilities of its European allies.

In 2014, NATO countries agreed to raise their defense spending to 2% of gross domestic product by 2024. So far, only five countries — mainly in eastern and central Europe where the threat of Russia looms large — have met that pledge.

Since his campaign days, Trump has demanded NATO countries meet that 2% figure, or even double it, immediately.

Germany, Europe's biggest economy, has expressed little interest in hitting that benchmark.

The metric of percentage of GDP spent on the military can also be deceptive. Defense spending has broad and differing definitions around the globe.

Greece is one of the few NATO countries that meet the 2% spending mark, but it spends much of that on pensions.

NATO's newest member, Montenegro, could spend 2% of its GDP on defense, which would be only $95 million, just over the cost of one US Air Force F-35.

NATO pullout?

Trump gave a speech to NATO members in Poland in July 2017 standing in front of a statue made of metal from the World Trade Center. Evan Vucci/AP

Trump on Monday also lamented the money the US has spent protecting other countries, saying the US gained nothing from the alliances other than "Deficits and Losses."

"It is time that these very rich countries either pay the United States for its great military protection, or protect themselves...and Trade must be made FREE and FAIR!" Trump concluded, appearing to wave the idea of a US pullout from NATO.

Article 5 of the NATO treaty, the alliance's key clause that guarantees a collective response to an attack on a member state, has been invoked only once in NATO's history: after the September 11, 2001, terrorist attacks on the US.

The result was a collective response from NATO countries that still have forces fighting and dying alongside US forces in Afghanistan today.