Nearly three weeks into what is in fact a full-on invasion, the reality for Russia’s troops looks more chaotic than anything else.

It’s much the same story as it was a fortnight ago: Terrible logistics, insecure communications systems, and a strong fight back from the Ukrainians have conspired to slow Russia’s advance to a crawl at best (at least, apart from in the east of the country). It may be trying to move on Kyiv, but it’s far from encircling or besieging it.

While it is very difficult to get accurate death tolls, U.S. intelligence estimates from five days ago put the number of Russian dead at around 6,000; military rule of thumb would suggest three times as many have been wounded and are therefore also out of action.

In the past 24 hours, British military intelligence reported that Russia is having to pull in additional troops from its far east and from Armenia to make up for these losses—as well as using mercenaries and Syrian fighters.

“Russia will likely attempt to use these forces to hold captured territory and free up its combat power to renew stalled offensive operations,” the Defense Ministry said, before issuing another update that noted Russia still hasn’t gained control of Ukrainian airspace (those missiles hitting Kyiv are probably being fired from Belarus).

“The tactics of the Ukrainian Armed Forces have adeptly exploited Russia’s lack of maneuver, frustrating the Russian advance and inflicting heavy losses on the invading forces,” the update read.

And then there are the dead Russian generals—four of them as of Tuesday, when Maj. Gen. Oleg Mityaev was reportedly killed by the Ukrainians.

“If you have generals at the front and putting themselves in dangerous positions, it’s often because they need to be there,” said Phillips O’Brien, professor of strategic studies at the University of St Andrews in Scotland. “It would normally be the result of having a lot of screw-ups they’re trying to deal with.”

O’Brien sees Russia’s estimated casualty tally as “extraordinary” for an advanced military power fighting a modern war. “The Ukrainians have fought really well, and the Russians were not prepared for this,” he told Fortune. “Their soldiers were not trained in the right way or were not emotionally prepared to encounter what they encountered.”

The professor is no fan of the theory that Russia has been holding back its more experienced and dangerous soldiers, arguing that “there was never any evidence for this” and the idea is an attempt to “explain away their badness.”

While Russia used its Assad-supporting role in Syria’s civil war to proudly demonstrate its military’s latest capabilities, O’Brien said that experience may have given them an “unrealistic expectation of what they could do” because unlike the Ukrainians, the Syrian rebels they were bombing “had no opportunity to fight back.”

So if this really is the best Moscow can muster, what does that mean for Russia’s chances in the coming weeks?

O’Brien reckons the thing to look out for is Russia’s ability to “adjust and learn” from their experiences thus far—for example, in how they deal with Ukraine’s Turkish Bayraktar drones, which are busy destroying Russia’s surface-to-air missile batteries.

“It shouldn’t be that difficult for [the Russians] to shoot down,” he said. “They have no defensive mechanism, they’re slow-moving—this shouldn’t be happening. If it keeps happening, then they’re not learning.”

For Russia to stand any chance of taking control of Ukraine, O’Brien suggested, it would need to rotate in a new army very soon.

“The army they have now has up to three weeks of high-level intensity left in it,” O’Brien said. “After those three weeks, people will need a break. If you force units that had six weeks of intense fighting to stay on the line, they’ll be terrible.

“If the Russian army doesn’t start moving soon, it’s really screwed.”