It's of monumental importance to the security of the world - and a thorn in Russia's side for generations. Donald Trump may be about to end it.
It's something Russia has been striving to achieve since the end of World War II: topple the NATO alliance.
The threat of nuclear armageddon. Espionage. Assassination. Information manipulation. All have been wielded by Moscow against the West with varying degrees of success.
But nothing has ever strained the strength of NATO as much as US President Donald Trump.
Now a former NATO supreme commander has called Trump's attacks the 'gift of the century' for Russian President Vladimir Putin.
And the revelation that the FBI initiated a formal probe into the possibility of Trump being a Russian 'asset' has revived fears about his true motives.
Trump's supporters insist their president is demonstrating his strength as a master deal-maker: forcing recalcitrant allies into paying their dues.
But his critics are cynical: why do his actions so often appear to achieve precisely what Putin wants?
Two recent US government reports have added substance to years of speculation.
Why is Trump so openly hostile and sceptical of a military alliance that has cemented America's world influence and deterred World War III for 70 years?
Why does he so openly admire such a blatant international rival as Putin?
The first official word of concerns over links to Moscow in his election campaign team only emerged after Trump was voted in on November 8, 2016.
It was revealed the FBI had already been given a dossier on Trump by a former British agent. It claimed Moscow had 'kompromat' — compromising material used for blackmail — on the new president.
FBI Director James Comey soon found his popularity with the president reversed. He had been lauded as a Republican hero for refusing to reveal the existence of the Trump dossier during the election campaign while delivering the damaging news Hillary Clinton was being investigated in the last days before the nation voted.
But Comey was fired within the opening weeks of the Trump administration. The president repeatedly stated this was, at least in part, due to Comey's refusal to drop the Russia investigation.
We've now learnt via the New York Times that this firing initiated a formal FBI investigation into the possibility Trump was a Russian agent. It wasn't long before this probe was passed on to Department of Justice special counsel Robert Mueller.
So far, some 33 of Trump's election campaign team and associates have been charged with a variety of crimes. Even his closest advisors, including his national security chief, his campaign leader and his former lawyer, have been interrogated over allegations Moscow attempted to influence the 2016 election.
Adding fuel to the fire is a weekend report by the Washington Post that Trump had gone to 'extraordinary lengths' to conceal the substance of his controversial one-on-one conversations with Putin. He held the meetings in private, without the usual advisors. He confiscated notes. He swore translators to silence. Even his closest aides were kept in the dark.
Trump has also repeatedly refused to criticise Russia on issues as crucial as its annexation of Crimea and actions against Ukraine.
"The net effect of this performance is that Putin can present his people with the fact that Russia isn't being held to account by the US president for shooting down the MH17 civil airliner or for the illegal use of a Novichok chemical weapon on UK soil that has seemingly killed a UK citizen," Australian Strategic Studies Institute analyst Michael Shoebridge wrote. "Worse than that, for his own confused personal and nakedly political reasons, Trump has told the world that he believes Putin's bare faced-lies about Russia not interfering in the US elections.
There's no doubt Trump is openly antagonistic towards Europe, and NATO.
It's nothing new. He's also questioned the value of alliances with South Korea and Japan. Without warning, he's pulled US troops out of Syria and decreed the winding down of operations in Afghanistan.
During his election campaign, he labelled the trans-Atlantic organisation "obsolete".
His visit to France in November last year for celebrations marking the centenary of the end of World War I quickly degenerated into farce.
NATO member states agreed in 2014 to lift spending on defence to a standard 2 per cent of GDPs before 2024. So far, five nations have met that goal.
Despite the obvious tensions, Trump has publically denied wanting to end the NATO alliance. In July, he said his commitment to NATO was 'very strong' and 'very important'.
But the New York Times has revealed the president last year repeatedly told his senior advisors that he wanted out. Importantly, during the NATO leader's summit in July, Trump reportedly said he could not see the point of maintaining the military alliance.
It was widely seen as an oblique criticism of the president's ambitions.
Now, Mattis is gone. And so have most White House advisors who viewed Moscow with trepidation.
His replacement, acting Defence Secretary Patrick Shanahan, now says the Pentagon should not be "the Department of No" when it comes to enacting Presidential desires.
Amidst it all, former supreme allied commander of NATO, Admiral James Stavridis, has said any weakening of the alliance would be "a geopolitical mistake of epic proportion".
"Even discussing the idea of leaving NATO — let alone actually doing so — would be the gift of the century for Putin," the retired admiral said.
Former Soviet KGB operative Putin wants to destabilise NATO. He wants to weaken the alliance opposing his ambitions.
But withdrawal from the alliance by the United States would achieve all this for him in one master stroke.
Putin flies into Serbia today after unleashing yet again on his European neighbours.
He has accused NATO of haiving "expansionist policies" and for destabilising the Balkans.
Serbia did not follow the rest of Europe in imposing sanctions on Russia over its actions in Ukraine. Belgrade also asserts it will never join NATO.
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"The policy of the United States and certain Western countries aimed to foster their dominance in the region constitutes a major destabilising factor," Putin told Servian news outlets earlier this week. Russia, however, "knows and understands the complexity of the Balkans and history of the region".
It's all about old loyalties. And new ones.
But Moscow itself has played a part in that destabilisation. Two Russian spies have been accused by Montenegro for attempting to organise a coup in 2016 to prevent it joining the alliance.
Few now doubt Moscow has returned to a state of Cold War with Europe, bringing to end the encouraging thaw brought about by the collapse of the Soviet Union.
Russia has seized territory from Georgia since 'intervening' in that independent state in 2011. Then President Putin annexed Crimea in 2014. He has since openly supported pro-Moscow insurgents in war-wracked Ukraine.
Putin sees NATO as a threat.
The Kremlin still regards these nations as belonging to its sphere of influence. And it's become increasingly aggressive towards the smaller, weaker NATO states of Latvia, Lithuania, Estonia on the Baltic coast.
"US President Donald Trump … has confirmed Europeans' worst fears: if another 'Crimea-like' take-over by Russia occurs somewhere on the continent, they will likely be on their own," Southern Cross University lecturer in international relations, Jean Renouf wrote.