"NATO and Moscow have suddenly remembered they used to be the world's best worst enemies," said a Brussels-based officer at NATO's headquarters, speaking on condition of anonymity.
"And our older colleagues are using the same reflexes they abandoned years ago," he added.
In NATO offices, maps of Ukraine are now pinned up on walls alongside that of Afghanistan, the country that most mattered until recently. "Russia experts are back in the centre-stage again," a diplomat added.
The mood has turned especially sour in the last days as Moscow and NATO each accuse the other of reviving the spirit of the Cold War that saw the United States and Russia in confrontational mode from 1947 to 1991.
"Nobody wants a new Cold War. Russia should stop trying to turn back the clock," Anders Fogh Rasmussen, secretary general of the Western military alliance, said this week.
But the Russian foreign ministry website instead pointed the finger at the NATO chief, accusing Rasmussen of "zealously replicating" Cold War rhetoric.
Moscow has reacted particularly bitterly to a vexatious decision taken end March by the 28 military allies to suspend NATO-Russian "practical cooperation" in reprisal for the annexation of Crimea.
This is because the move amounts to a snub in practical terms, meaning that Russians no longer have easy access to NATO headquarters -- bar just four of them, including the Russian ambassador to NATO.
"Introduction of these restrictive measures against the diplomats confirms yet another time that the North Atlantic alliance is incapable of rising above the mentality of the Cold War era, and prefers the language of sanctions to dialogue," said Russia's foreign ministry.
- 'Chaotic Relations' -
The latest clash occurred Thursday when NATO released satellite photos to prove the presence of a massive and "very capable" Russian force massed along Ukraine's border.
Unrolling its campaign on social media such as Twitter, NATO put out black-and-white shots showing hundreds of tanks and vehicles along with scores of aircraft parked in the countryside near the frontier.
Russia responded rapidly, saying the pictures dated back to the summer of 2013, not to March and April as stated by NATO. On Friday, the alliance nixed those claims with a fresh batch of pictures "in order to clearly show that the claims of Russian officials are categorically false".
Russia-watchers and defence specialists are taking in events with a pinch of salt.
"These references to the Cold War are a heresy both on the geo-political level and militarily," said Philippe Migault at the Paris-based Institute of International and Strategic Relations (IRIS).
"Putin's Russia does not entertain the same global ambitions as the Soviet Union" and "is not directly threatening any NATO nation", he said.
Moreover "there is absolutely no longer a balance between the two blocs" given that the military budget of NATO countries nowadays is around 12 times bigger than Russia's.
Though the Ukraine crisis is clearly the worst moment in NATO-Russia ties since the fall of the Soviet Union, it is not the first.
NATO's eastward expansion embracing the Baltic nations, Poland and Romania was a cause of tension as was the Kosovo crisis and moves to build an anti-missile shield in Europe.
Nonetheless cooperation was on the march, notably in Afghanistan and in the fight against maritime piracy.
"NATO-Russia ties have always been chaotic and there's no reason to think things will change," said a Brussels diplomat.