Editor's Note: (Matthew Chance is CNN's senior international correspondent. He has reported extensively on major stories for CNN's global news networks from the Middle East, Afghanistan, Russia and Chechnya, Europe and the Far East. The opinions expressed in this article belong to the author.)
(CNN) The catastrophic breakdown of the Syrian truce, less than a week after it came into force, has pushed relations between Russia and the United States to a new low.
Not since the end of the Cold War has the rhetoric between Moscow and Washington -- who back opposing sides in the conflict -- been so acute.
Both blame the other for failing to rein in their Syrian proxies.
Moscow was furious at an attack last weekend by warplanes of the US-led coalition on Syrian Army positions, which they say killed more than 60 personnel.
US officials say the strike, near Deir Ezour in Eastern Syria, was a mistake; a case of friendly fire.
The US says they thought they were ISIS.
But the Russian foreign ministry rejected that outright, calling an emergency session of the UN Security Council and issuing an extraordinary statement, in which it accused the US of colluding with Islamic State rebels in Syria.
But it was an appalling incident in Syria a few days later that delivered a near fatal blow to peace efforts, and sent US-Russian relations further into a tailspin.
As it waited to deliver its cargo of blankets, food and other humanitarian aid to the suffering people of Aleppo, a UN convoy came under devastating attack.
Eyewitnesses spoke of barrel bombs being dropped from the air and of jets circling in the night skies above.
The US said all signs pointed to a Russian air attack, but Russia insists neither its air force nor that of its Syrian government ally carried out the attack, which has been condemned by human rights groups as a war crime in which at least 20 civilian aid workers were killed.
"I think we need to refrain from emotional reactions and from making comments immediately," Russia's foreign minister Sergei Lavrov implored the UN in New York.
"We need to investigate and be very professional," he added.
Moscow had already suggested alternative explanations for the attack, saying the aid convoy may have been set ablaze by rebels, or even that a US Predator drone could have been responsible for obliterating it.
It all seemed too much of a stretch for the exasperated US secretary of State John Kerry, who had spent months brokering the Syria truce with Russia, and who now launched a blistering verbal assault on his Kremlin counterpart.
"I listen to my colleague from Russia and I sort of felt a little bit like we're sort of in a parallel universe," he raged.
"When you've signed up to a ceasefire and you don't adhere to it. What kind of credibility do you have, with any of your people?"
The diplomatic clashing of swords over Syria, of course, did not come out a vacuum.
Russia and the US have been at loggerheads for years over a range of foreign policy issues: from the Gulf war in 1990 to the Arab spring uprisings that began in 2010.
Add to these NATO expansion and US missile defence and you get a sense that even though the Cold War ended, the mutual rivalry and suspicion didn't.
The latest round of US-Russian tensions began in 2014, when a popular pro-Western uprising toppled the Ukrainian government.
The Kremlin responded by effectively invading, then annexing, the strategic Crimean Peninsula where Russia leased a naval base for its Black Sea Fleet.
It then fueled a rebellion in Eastern Ukraine, arming rebels and even sending in Russian Army units to fight, according to US officials.
The US response was one of outrage, and economic sanctions on Russia were soon to follow, cementing the old, familiar bitterness between these two former foes.
At the heart of Russia's rocky relationship with the US stands Vladimir Putin, the man who has led the country as President, then Prime Minister, then President again, for the past 17 years.
It's his muscular foreign policy, forcing the world to recognize Russia as a major power once again, that has so angered the West and made him increasingly popular at home.
Despite a tough economic climate, battered by low oil prices and sanctions, he still gets approval ratings of which the US President can only dream.
True, recent Russian parliamentary elections saw a relatively low turnout. But that is not necessarily a reflection of Putin's popularity. He was not, after all, standing for election himself.
Even in American politics, the Putin Factor has become a major campaign issue, amid startling allegations -- denied in Moscow -- that the Kremlin is trying to influence the outcome of the presidential vote.
The influence of the Russian leader is hard to ignore.
Indeed, it may be this same Putin Factor that decides what happens next in the bloodbath of Syria.
Secretary of State Kerry, in his emotional plea at the UN this week for a renewal of the ceasefire, said this is a "moment of truth."
He may have been addressing the entire UN Security Council in that moment, but in reality, he was speaking to just one man.