The NATO summit celebrates its 70th birthday in London this week but some of the main attendees aren't quite in the party mood.

The biggest guest, President Donald Trump, has traveled to the U.K. capital to attend the meeting and join other heads of state in marking the alliance's seven decades in existence.

Trump has repeatedly said the U.S. provides too much cash for NATO, spending big on maintaining missile defense systems across Europe and positioning 65,000 troops within the continent. On Tuesday, Trump issued another broadside, this time accusing Germany of not paying its fair share on defense.

A withdrawal of the U.S. from NATO would effectively destroy it with one stroke, and while nothing official has ever been said, several reports suggest Trump has considered ending U.S. involvement.

NATO itself estimates that as a percentage of GDP, the U.S. will far outstrip the spending of any other member country in 2019.

In 2014, all NATO members agreed to increase their defense spending to 2% of gross domestic product by 2024.

As of June 2019, NATO data estimated that only seven of its 29 members — including the U.S. — are estimated to spend 2% or more of their annual GDP on defense this year.

On Monday, Secretary General Jens Stoltenberg defended the spending of member nations.

"On spending, NATO allies are now really stepping up and delivering more that they have done for many, many years," he told CNBC's Hadley Gamble in London.

Stoltenberg said non-U.S. NATO allies, i.e. Europe plus Canada, are on target to add more than $400 billion to their defense budgets by 2024.

"That's unprecedented and will make NATO stronger," he added.

Should that additional spending materialize, it will bring the combined spending of all the NATO allies to around par with the current U.S. outlay alone based on GDP.

NATO's earliest incarnation was a 1947 agreement between France and the U.K. to help each other in the event of any attack from Germany or the Soviet Union.

This expanded into a military alliance that included the U.S.. In 1949, member countries put their signature to the North Atlantic Treaty for the first time.

Its relevance and popularity has ebbed and flowed. In 1966, France, under President Charles de Gaulle, left NATO's military structure, doubting the organization's might against any invasion from the Soviet Union.

Full membership by France was restored only in 2009 and just 10 years later, current French leader Emmanuel Macron is again doubting the alliance.

Macron has said NATO is experiencing "brain death" because of pressure to reform from Trump and unpredictable military action from Turkey.

Macron has said the Turkish invasion of Syria is a threat to NATO's battle against the Islamic State.

In an angry response, Turkish President Recep Tayyip Erdogan said Macron himself was suffering from "brain death" and he was showing disrespect and hubris by questioning Turkish action.

"You know how to show off, but you cannot even properly pay for NATO. You are a novice," Erdogan reportedly said.

NATO's Stoltenberg said Monday that while the members have often had different opinions, history shows that "we have always been able to agree around our core task to protect and defend each other."

As Trump, Macron and Erdogan meet, this week will provide a stern test of that theory.