The vogue word "game-changer" has been heavily overused in recent months. But the agreement that seems to have emerged on Syria from more than five hours of intensive talks in Moscow between US Secretary of State John Kerry, Russian President Vladimir Putin and Russian Foreign Minister Sergei Lavrov may turn out to be just that.
For the past two years, the international community has been hamstrung from taking concerted action over Syria, because the UN Security Council was paralysed by big power differences, with disputes between Washington and Moscow at the core.
Now at last the two seem to be genuinely singing from the same song sheet.
That sets one of the parameters that are essential for successful movement towards a settlement of the Syrian conflict, which has embroiled many outside players in a complex struggle that is already tearing the country apart and plunging the region into turmoil.
Arresting and reversing that process of disintegration is a massive task, not least because of the diverse array of forces engaged in the free-for-all.
The international and Arab League envoy, Lakhdar Brahimi, stressed that, vital and hopeful as the apparent US-Russian understanding is, it is only a first step.
In theory, the international powers agreed on the modalities for a settlement at a meeting in Geneva on 30 June last year, which called for the formation of a fully-empowered transitional government.
But in practice, sufficient ambiguities were left open in the Geneva statement for the bickering to start almost before the ink had dried.
In particular, there was no understanding on the future role of President Bashar al-Assad and his inner circle.
Many opposition players insisted that he should stand down before any negotiations, and the Americans - casting round for a plausible policy - appeared generally to back that demand.
Now Washington seems to have softened its position to the extent of leaving Mr Assad's future up to the outcome of negotiations and whatever the Syrians themselves decide, which has long been the Russian position.
That compromise offers a chance of pulling back from the abyss into which everyone is already staring, if not already sliding.
The alternative is a scenario which does not serve the interests of either of the big powers, and it is hard to see how it serves anybody else's.
Which is not to say that it will not happen.
Democrats in the US Congress have introduced "Syrian stabilisation" legislation that would empower the administration to provide lethal aid to the Syrian opposition - weaponry that could tilt the balance on the ground, which would be the purpose.
But Mr Kerry made it clear that that scenario would become irrelevant if there is a serious settlement process.
The bottom line is that the US administration does not want the rebels to win.
Its strategy was to tilt the battlefield sufficiently that the government - or the Alawite leadership - came under enough pressure to jettison the ruling circle and agree to regime change through an orderly transition.
But that was not happening, and the risk attendant on beefing up support for the rebels and prolonging the conflict is that it could lead to an uncontrolled regime collapse and chaos, with all kinds of radical groups possibly moving in.
The past few months have seen reflected on the ground the fact that Russia, Iran and the latter's Lebanese allies, Hezbollah, are determined not to allow the Syrian government to be brought down by force.
With government forces making increasing headway against rebels in many areas, the US would have had to put huge resources - and perhaps direct involvement - into redressing the balance and tilting it the other way, risking embroilment in a deepening proxy war that could go badly wrong, and would actually stand little chance of going right.
That prospect was clearly not attractive for the Russians either.
The 30 June Geneva statement is destined to provide the starting point for the conference the two big powers propose to convene in the coming weeks.
This time, they seem genuinely committed to working together to bring their allies to the table for a constructive negotiation, rather than agreeing a lowest common denominator text that would get torn to shreds immediately by unresolved disputes.
But it will not be easy, on either side of the equation.
A successful negotiation is normally based on translating an established balance of power on the ground into political reality, and obviously depends on the parties involved being able to deliver their side of the deal.
In this case, the balance of power on the ground is fragmented and constantly shifting.
Nor is it clear who would talk on behalf of whom.
On the government side, the picture is fairly cohesive, though any negotiation would need the opposition to agree to sit down, or deal indirectly, with the powers that be.
On the opposition side, the situation is far less clear.
The umbrella Syrian National Coalition has signally failed to establish itself as a coherent, unified and focused force to be reckoned with.
After months of internal wrangling it has not even been able to produce the transitional or provisional government that the Western powers were hoping would provide a credible vehicle for regime change.
But the coalition has received limited international recognition as the sole legitimate representative of the Syrian people, which it may try to invoke in arguing that it alone should take part in negotiations, to the exclusion, for example, of government-tolerated internal opposition groups.
Whoever talks for the opposition, there is also the issue of whether they can deliver.
The coalition is largely based on opposition figures living abroad.
The fighting forces on the ground are desperately fragmented, in some cases very localised, in others, involving radical Islamist jihadis from outside the country.
Who is going to deliver - or force - compliance by the al-Qaeda-related al-Nusra Front, which has made the running in many rebel-held areas?
A negotiation requires both sides to concede that they have not won and that a compromise is needed to save the country.
The opposition would be implicitly admitting that, if they agree to talks without President Assad stepping down.
That is still a big step away, to judge from a statement issued by the opposition coalition after the US-Russian talks, reiterating that "any peaceful solution requires the immediate removal of Bashar al-Assad and the heads of his security apparatus; any solution that does not include these elements is rejected at the political level and by the people of Syria".
But the Syrian government must also agree to a serious negotiation and deep-cutting compromises. Its willingness to do so is by no means assured and may depend on Russian resolve.
For months, the government has been volubly engaged in what it calls preparations for a "national dialogue on a political solution" for the crisis.
That has not been taken seriously internationally. The regime has never used the word "negotiation", with all that it implies, but has proposed a "dialogue under the national roof" - in other words, opposition figures who are ready to give up the struggle are welcome to come to the government's table and join a discussion which might lead to minor cosmetic tinkering with the power structure, but would be unlikely to produce radical change.
With the Syrian army currently on the advance in many areas and the rebels looking less likely to win than for some time, elements of the regime may still be tempted to think that a military victory is possible.
That is where Russian influence will be crucial in persuading Damascus that serious change must be enabled and that there can be no simple turning back of the clock.
While all the outside parties pay lip service to the notion that any settlement has to be arrived at by the Syrians alone and not dictated by external powers, the fact is that entente - or discord - between those powers will be crucial to success or failure of a solution.
Which raises the crucial question of the endgame: what kind of Syria do they envisage emerging from all this?
Can a formula be found that would be acceptable to the Americans, the Russians, and indeed the Iranians? And, for that matter, Israel?
Syria under the Assad dynasty since 1970 has been a staunch ally of Moscow - and is now Russia's only real Middle East friend - and a key component, with Iran and Hezbollah, of the "axis of resistance" to Israel and its Western backers.
Since Hezbollah was set up in Lebanon by Iran and Syria to counter the Israeli invasion and occupation in 1982, arms have routinely been making their way to Hezbollah from Iran via Syria.
Are those days now over?
The devastating and spectacular Israeli air strikes near Damascus in the early hours of Sunday 5 May may have been intended to serve notice not just to Syria and Iran, but also to Washington and Moscow, that Israel would not permit that channel to continue, either for special delivery of advanced weapons, or for routine supplies.
Acceptance of that would imply a huge shift in Syria's future orientation and relationships, and in the regional strategic structure and balance.
Can the Syrian government be induced to accept that? Would the Russians themselves accept it, and what about Iran and Hezbollah? And would the Americans and Israelis accept anything less?
It is hard to see those issues being resolved, which they must be if there is to be a viable strategic framework for a new Syria.
It is equally hard to imagine what kind of internal balance can be struck that would produce a stable governance structure out of the current chaos.
How much of the existing security and military structures would be carried over, and how much of the ruling Baath Party's apparatus?
Would real democracy and truly free and fair elections be the objective, with the attendant possibility - even likelihood - that, as in Egypt and elsewhere, the Muslim Brotherhood would emerge as the dominant political force, firmly rooted as they are in the majority Sunni community?
Is that what the US, Russia and the other players really want?
And if not that, then what?
There are no easy answers, and many other questions.
That is why Lakhdar Brahimi was right to stress that the Moscow agreements were just a first step.
Many more must follow, and there will be problems all the way.
But everybody knows what the alternative is, because it is already pretty much there.