Vladimir Putin has bet his own and his country’s future on starting the biggest war in Europe for generations.
Xi Jinping, probably the second most powerful individual on the planet, could have a profound influence on its outcome.
But can he end it? And does he really want to?
Those were the questions on everyone’s mind as the Chinese president landed in Moscow yesterday morning.
Direct talks between Ukraine and Russia stalled months ago, and it currently seems impossible to imagine either side making a concession that the other would accept.
There are two ways Mr Xi could cut that Gordian knot.
The first is to tell Mr Putin over their sturgeon soup dinner to get real, accept he can never win the war, and that it is time for a “face-saving” withdrawal.
This is the message Ukraine and its Western allies, including the UK, have publicly called on Mr Xi to deliver.
For any chance of success, he would have to be prepared to do some brutal diplomatic arm twisting.
The Russians would have to make a significant and painful concession, probably involving giving up much of the land they have occupied, to make it work.
Mr Xi would probably also need the Ukrainians to give up some of their stated war aims, probably starting with Crimea – but he has less influence there.
Now, Mr Xi wields unique influence in Moscow, and China has lately shown surprising skill as a peace broker.
His government this month pulled from its hat the diplomatic rabbit of a rapprochement between Saudi Arabia and Iran. But it is difficult to imagine any stick or carrot he could bring to Moscow that others have not already offered.
The second option is to come off the fence, accept this war will be decided on the battlefield, and give Mr Putin what he needs to win.
That would mirror the West’s own stated aim of putting Ukraine in a better negotiating position when the time for talking comes.
But it would also mean jumping with both feet into the quagmire Mr Xi has been tip-toeing around for a year.
It would jeopardise China’s relationship with its biggest trading partners, put it in a direct proxy war with the West, and sink its credibility as a non-aligned actor.
Nor is there any guarantee it would work.
Given these obstacles, one could be forgiven for suspecting Mr Xi’s has no intention of attempting the impossible.
Last month, China’s foreign ministry published a 12-point road map to peace.
It calls for a resumption of peace talks with the goal of de-escalating tensions and eventually achieving a ceasefire – but offers no concrete road map for doing so.
It talks about sovereignty while making no mention of a Russian withdrawal, to the irritation of Ukraine.
Its only explicit proposals – endorsing the grain deal to keep global food supplies flowing, condemning nuclear weapons use and threats thereof, backing the IAEA to secure civilian nuclear power plants – are those everyone already agrees with.
So perhaps, behind the warm rhetoric about unlimited partnerships, Mr Xi doesn’t really intend to do much at all to help his ally.
When he waves Mr Xi goodbye tomorrow, Mr Putin will hope to have secured China’s backing to finish the war on Russian terms.
But he may find himself less certain of his ally’s intentions, and wondering just what the point of this partnership is.