NATO, the 29-member military alliance, was set up 70 years ago to counter the threat posed by the-then Soviet Union.
But now, another rising military power is in its sights: China.
As heads of state and government gather in the U.K. Tuesday for a two-day meeting of the alliance, shifting geopolitical relationships and emerging challenges will be in focus for the fractious group. Previous meetings have been dominated by the alliance's old foe Russia, following Moscow's annexation of Crimea from Ukraine in 2014.
However, many experts and leaders within the group think the alliance should now be focusing on new and emerging military powers, like China.
NATO Secretary General Jens Stoltenberg told delegates at the "NATO Engages" event in London, a precursor to the "Leaders Meeting" that starts Tuesday evening, that for the first time China was on NATO's agenda.
"The rise of China has security implications for all allies. There are some obvious opportunities but also some obvious challenges," he told the audience, adding that allies needed to find a "balanced way" to respond to the challenge posed by China.
Nonetheless, he said there were no plans for a "NATO-China Council," similar to the NATO-Russia Council (NRC) that was set up in 2002 to improve dialog and cooperation between NATO and Russia.
"What we see is that the rising power of China is shifting the global balance of power and the rises of China — the economic rise, the military rise — provides some opportunities but also some serious challenges," Stoltenberg told CNBC's Hadley Gamble in London.
"We have to address the fact that China is coming closer to us, investing heavily in infrastructure. We see them in Africa, we see them in the Arctic, we see them in cyber space and China now has the second-largest defense budget in the world," he said.
According to NATO's own estimate, China had the second-largest global defense budget in 2018. In March, China set its 2019 defense spending at 7.5% higher than a year ago, raising spending to 1.19 trillion yuan ($177.61 billion), according to known figures (some believe the actual figure could be higher).
Still, it lags a long way behind the U.S. In April, the U.S. Defense Department asked Congress for $718 billion in its fiscal 2020 budget, an increase of $33 billion or about 5% over what Congress enacted for fiscal 2019.
The U.S. and NATO are watching China closely. Stoltenberg told CNBC Monday that NATO did not want to "create new adversaries" and that "as long as NATO allies stand together, we are strong and we are safe ... We are by far the strongest military power in the world."
Sino-U.S. tensions are of course already high as a trade dispute between the nations, which has led to billions of dollars' worth of import tariffs on each others' goods, remains unresolved.
U.S. Permanent Representative to NATO, Ambassador Kay Bailey Hutchison, told CNBC Monday that the rest of the world had let China get by with not meeting World Trade Organization (WTO) standards, but that it was now time to bring China "into the rules-based order."
"They have now turned into a competitor, but they still expect to have the acquiescence to not abiding by the rules, to stealing technology and intellectual property," she said before adding, "We have let them get by with it, now we can't. They are a competitor. A very strong competitor."
Bailey Hutchison said NATO is now reviewing Chinese military action due to that rapid growth. The ambassador said being watchful is both new and totally justified. "We have to face that, and we have to see it clear-eyed. Do we want China to be an enemy? No, we don't, but we must prepare," she said.
The ambassador said it was not yet the time to declare China an adversary and that efforts should remain on making the country a strong and fair trading partner.