The sight of Russian troops in headlong retreat in Ukraine is stunning — but it should not be surprising.
This war has gone badly for Russia from the outset. Vladimir Putin failed to achieve the lightning victory that he was aiming for on February 24. By April, the Russianshad been forced into a humiliating retreat after making incursions towards Kyiv.
The limited gains Russia has made over the past six months have come at a terrible cost. The original invasion force mustered by the Kremlin was around 200,000 troops. The US estimated last month that 70,000-80,000 of that force has been killed or wounded since the beginning of the invasion.
Unwilling to acknowledge that Russia is at war, Putin has refused to institute conscription. By contrast, Ukraine has mobilised its entire adult male population. As a result, Ukraine now probably has more troops on the battlefield than Russia.
The Ukrainians also have the advantage in morale and munitions. They are fighting to defend their own country. The supply of advanced weaponry from the US and Europe — in particular, accurate long-range missiles — means they are now better equipped than the Russians.
The prospect of Russian defeat is real and exhilarating. But Ukraine's advances also open a new and dangerous phase in the conflict.
The pictures of weeping civilians embracing Ukrainian soldiers as they liberate towns and villages from the Russians underline what this war is all about. Permanent Russian occupation would snuff out political freedom and would be enforced with killings, torture and deportations.
An easy Russian victory in Ukraine would also have opened the door to further aggression against its neighbours — including Moldova and perhaps even Nato members Estonia, Latvia and Lithuania. That prospect was alarming enough to persuade Finland and Sweden to apply for Nato membership.
If Russia is defeated, the invasion threat hovering over the rest of Europe will recede. The global political atmosphere will also change. Russian defeat will go down badly in Beijing and Mar-a-Lago. In the weeks before the invasion, China announced a friendship "without limits" with Russia. Donald Trump chortled that Vladimir Putin was a "genius". That judgment now looks not just immoral, but stupid.
But some caution is in order. Almost a fifth of Ukraine is still occupied. The Russians will try to regroup and the Ukrainians could over-reach.
The really complex question is what happens if Russia is facing a humiliating defeat — perhaps including the loss of Crimea, which was occupied in 2014 amid much rejoicing in Moscow?
Putin's options
Rather than accept defeat, Putin may try to escalate. His options, however, look limited and unappealing. The refusal to call a general mobilisation must reflect nervousness about the opposition that could stir in Russian society. Calling up troops, training and equipping them will take many weeks — and the war is moving fast.
From the beginning of the conflict, Putin has hinted that Russia might use nuclear weapons. The White House has always viewed this possibility seriously. As the war has dragged on and gone badly for Russia, fears that Putin might resort to nuclear weapons have receded a little, but they have not gone away. As one senior Western policymaker put it to me last week: "We have to remember that almost every Russian military exercise we've observed has involved the use of nuclear weapons."
Using nuclear weapons in Ukraine would, however, create the obvious danger that Russia itself would be contaminated by the fallout. The global political reaction would be very negative and a Western military response, probably non-nuclear, would be all but inevitable.
Winter is coming
Like Russian leaders in the past, Putin is hoping that winter will come to his rescue. Russia's recent announcement that it will stop almost all gas supplies to Europe is clearly intended to freeze the Western supporters of Ukraine into submission.
But Putin needs a lot to go right for the gas gambit to work. A very cold winter or a surge in political protests in the West would help. Neither can be relied upon. The German government says the country "is now better prepared for a halt to Russian supplies" and that the total gas storage level is almost 87 per cent. Energy price subsidies are being rolled out across Europe.
So the Russian leader's position looks perilous. From the start some Western leaders have quietly hoped that Putin would lose power as a result of the war. President Joe Biden even blurted it out.
But if Putin is deposed, perhaps by a palace coup, his replacement is more likely to be a hardline nationalist than a liberal. The most vocal dissent being expressed in Russia is from militarists and nationalists — calling for escalation of the war. One theory doing the rounds in Western intelligence circles is that the murder of Daria Dugina, a nationalist journalist, was organised by the Russian security services as a warning to Putin's ultra-right critics.
A defeated Russia would not disappear off the map. And it would still possess large numbers of nuclear weapons, as well as a replenished stock of grievances.
So many dangers clearly lie ahead. But sometimes good news has to be recognised for what it is. In what has been a bleak year, the Ukrainian military victories of the past week are certainly that.
• Gideon Rachman is chief foreign affairs columnist for the Financial Times