It's nothing like the great breakthroughs of the mid-20th century wars, when combined air and ground forces would tear a hole in the enemy line, the tanks would pour through, and the front would roll back several hundred kilometres before it stabilised again.
The breakthrough in Ukraine is happening now, but in slow motion. Even on the fastest-moving front, in western Zaporizhzhia, it has taken the Ukrainian infantry 10 weeks to advance 10 kilometres through the dense and heavily defended Russian minefields -- not much faster than the British army at the Battle of the Somme in 1916.
It's moving so slowly because the Ukrainians do not have air superiority. (Sometime next year, perhaps, when the West finally gives them 40-year-old F-16s.) The ubiquitous Russian drones also slowed them down: almost all the mine-clearing had to be done at night, by hand.
But the Ukrainians have finally broken through the main Russian line just to the west of the fortified village of Verbove, and they have expanded the breech wide enough to start moving heavy equipment through it.
There are further, less well-built Russian entrenchments behind this line, and even more trenches are being dug farther south right now, so don't imagine Ukrainian tank columns racing across the landscape. Unless the Russian army collapses, it's never going to be like that again.
But the Ukrainians will now be able to advance faster -- a few kilometres a week, perhaps -- until the rasputitsa (the rain and mud season) arrives sometime next month and stops all off-road movement for vehicles until the winter freeze-up.
That will slow the Ukrainian advance down again, but it probably won't stop it because their foot-soldiers can still move across country. They won't reach the Black Sea coast this year and physically cut off all the Russian occupation forces west of there, as they once hoped -- but they may get far enough to starve the Russians.
Ukraine's HIMARS rocket artillery can already reach the only railway line connecting Russia with its forces in the western parts of occupied Ukraine. If they advance just another 15-20km, they will also be able to hit the main road west along the Black Sea coast with HIMARS.
Once Washington delivers the promised ATACMS long-range (300km) missiles for use with the HIMARS, Ukraine will also be able to take out the Kerch Bridge from Russia to occupied Crimea. (The western half of the bridge is on Ukrainian territory, so Kyiv would not be breaking its promise to US President Biden not to strike targets in Russia with US weapons.)
At that point, perhaps before the end of this year, around half of the Russian troops in Ukraine would be more or less cut off, on half-rations for supplies and ammunition at best. This is by no means certain to happen, but it is a realistic possibility.
Russian generals will be aware of this danger, but although Russia has more than three times Ukraine's population, its troops in Ukraine barely outnumber the Ukrainian defenders and they are close to exhaustion.
A mass mobilisation of Russian reservists might help, but President Vladimir Putin still seems determined to avoid that for fear of provoking a popular backlash against the war. Besides, general mobilisation would take six months to have any positive effect at the front, so it may already be too late for it to restore the situation.
Winning this war is not vital for Russia. It's just a foreign military adventure gone wrong, like Britain and France invading Egypt in 1956 or the United States invading Afghanistan and Iraq in 2001-2003.
However, winning this war is vital for Mr Putin's personal position and perhaps even for his life, so there will be a point of maximum danger if and when he finds out he is going to lose. If he should decide to gamble everything on a potentially catastrophic nuclear escalation, would the people around him go along with it?
There is a lot of apocalyptic talk among the Russian elite right now, but this is the original homeland of 'doublethink' (in George Orwell's 1984). Orwell never visited Russia, but he got it exactly right: all those Russian talking heads who are currently saying that the survival of Russia is at stake also know that it's not really threatened at all.
Which means they are unlikely to risk their families' lives just for Mr Putin's survival.
Gwynne Dyer is an independent journalist whose articles are published in 45 countries. His new book is 'The Shortest History of War'.