It has been a tradition since the days of Winston Churchill that the prime minister's address at the annual Lord Mayor's Banquet in London is devoted to foreign policy.
For the past four years David Cameron has argued that the bed-rock of this should be economic self-interest - Britain's foreign policy needed a new "hard headed" commercial focus, to seek out new trade and investment partners round the world.
Among those new partners was Russia.
Differences with Russia should be addressed candidly, said Mr Cameron in 2011, but not be allowed to define or limit relations with Britain.
In other words, doing business with Russia trumped all.
Now he is arguing the opposite - when rules were breached, he said this week, then Britain could not just draw a line for fear its economy would suffer.
The new mantra is that standing up for values is as important, and sometimes more important, than pursuing economic gain.
This wriggle to accommodate a shift in priorities was underscored in the centrepiece of David Cameron's speech - a withering attack on Russia.
What the Russians were doing in Ukraine was "illegal", he said, destabilising it and violating its territorial integrity.
"They are ripping up the international rule book," he said.
He noted that recent days had seen further shelling in south-east Ukraine and reports of more heavy weapons being moved in from Russia and concluded that "Russia's actions pose a grave danger to the rest of Europe".
This rhetoric is not entirely new. Mr Cameron has used tough language about Russia's actions in Ukraine before.
Even the phrase "ripping up the international rule book" is not freshly minted - look back to the joint article he penned with US President Barack Obama ahead of the Nato summit in September.
Nor is it the first time he has tendered a comparison between Russian President Vladimir Putin's actions in Ukraine and Hitler's behaviour on the eve of World War Two.
Downing Street tells us that it was this language the British prime minister used round the table at EU summits this spring and summer to make the case for sanctions.
But this time his blunt accusations were levelled in public.
"Here in this building - with its history shaped by the Blitz - we shouldn't need to be reminded of the consequences of turning a blind eye when big countries in Europe bully smaller countries," he said to 1,000 or so guests gathered amidst the medieval finery of the Great Hall of Guildhall, the power house of the City of London.
So why lash out at Russia now?
Well in part, it seems, because of the chilling warning that came from Mikhail Gorbachev at a recent symposium in Berlin, which has - because of who he is - set nerves on edge across the continent.
"The world is on the brink of a new Cold War," said the former Soviet leader, adding: "Some are even saying that it has already begun."
And in part because this Guildhall speech was by way of a public rehearsal for what Mr Cameron tells us he wants to say to President Putin when he meets him face to face at the G20 summit in Brisbane this weekend.
For a while it was not clear the Russian president would be going to Brisbane.
After all, in the wake of Western sanctions, he had been disinvited from attending - or hosting - G8 meetings.
Tony Abbott, the Australian host, had mused aloud whether it was appropriate for him to come.
But the G20 summit is an international gathering, convened by members who do not all share the West's antipathy for Russia's president.
Vetoing the guest list is not in the Australian prime minister's gift.
Plus this does present an opportunity for Western leaders to eyeball Mr Putin and see whether there is any way that this Ukrainian crisis - which seems to be fast unravelling on the ground - could somehow be stopped in its tracks.
But don't hold your breath for peace to break out.
Already, apparently, there have been tetchy meetings between the Russian president and his American and Australian counterparts at this week's earlier Asia-Pacific summit in Beijing.
And if Mr Cameron is anything to go by, Brisbane will deliver even more fractious face-offs to stoke tension, not tone it down.
In his speech in London Mr Cameron said he did not believe a new Cold War was inevitable or desirable.
But nor, he added, would Britain step aside from further confrontation with Russia if it did not change its course.
Sanctions on Russia, argued Mr Cameron, were having an effect - the rouble had plummeted, and capital was flowing out of the country.
There is no denying this - even Russian officials admit the economy is hurting.
This week Russia's Central Bank revised upwards its estimate for capital flight this year from an already record $90bn (£57bn) to an even more staggering $128bn.
What Mr Cameron will say to Mr Putin, apparently, is that Britain is prepared to tighten the screws still more.
"If Russia continues on its current path, then we will keep upping the pressure," he said, "And Russia's relationship with the rest of the world will be radically different in the future."
No prizes for guessing what Mr Putin, hot foot from glad-handing Asian and Latin American leaders in Beijing, will say to that - Russia has plenty of other options and other partners to turn to. It does not need the West.
And do not expect the Kremlin leader to give an inch.
Look no further than his scathing assessment of US foreign policy over the past 20 years in his recent Valdai speech, and his warning that worse relations will follow, unless the West agrees to a new partnership on Russia's terms.
Loss of trust
And just in case Western leaders did not clock that Valdai message, it was echoed in Berlin by Mikhail Gorbachev.
If anyone was to blame for the loss of trust that had led to the current crisis, said Mr Gorbachev, then it was the West and particularly the US.
They had declared victory in the Cold War and let euphoria and triumphalism go to their heads.
Powerful words from the last Soviet president, an international statesman for years feted in Western capitals for his part in bringing the Cold War stand-off to an end and for his fearless attacks on Mr Putin for undermining Russia's faltering democracy.
But these days Mr Gorbachev no longer criticises the Russian president - he echoes him instead.
The absence of trust and lack of a viable compromise in this conflict is sending the two sides hurtling in opposite directions. The rhetoric is hardening.
In theory Russia and the West still share common interests - in securing a nuclear deal with Iran, in containing the threat in Syria and Iraq from the ruthless fighters of Islamic State, in finding a way to stop the low level conflict in eastern Ukraine from turning into a full blown war.
Possibly private deals could still be done.
But there is precious little optimism to be garnered from the public posturing offered from the podium over the past few weeks.