The Russian soldier was angry, cold and tired. His brigade had fought for weeks on the outskirts of Kyiv. Half of them were injured or dead. So, in the middle of yet another torturous battle, he seized an opportunity. He repositioned his tank and ran over his commanderIn total, seven Russian generals and at least 15 senior Russian commanders have been killed in Ukraine in just one month of war. . 

Reports about Colonel Yuri Medvedev's condition are unclear. Some say he died of his wounds. Others say he was stretchered away to a hospital in Belarus, alive but with serious leg injuries. 

One thing is certain: Vladimir Putin's war in Ukraine is not going according to plan. "It increasingly looks like Putin has massively misjudged the situation,"  GCHQThe Government Communications Headquarters is a British intelligence and security organisation.  director Jeremy Fleming said in a rare speech. 

"We've seen Russian soldiers - short of weapons and morale - refusing to carry out orders, sabotaging their own equipment and even accidentally shooting down their own aircraft." 

Losses on the battlefield are mounting up. At least 15,000 Russian troops are thought to have died. Others have begged civilians for food, abandoned their vehicles and even defected to the Ukrainian army. 

US intelligence officials believe military leaders are too scared to tell the Russian president just how badly the war is going. Now, some are wondering: could these strategic failures be the catalyst that finally topples Putin's regime? 

Historians say there are two main ways authoritarians fall: military coups or popular uprisings. "Before the war, the risk from either of those threats was close to zero," argues academic Brian TaylorTaylor is the author of The Code of Putinism. . "And now the risk in both of those respects is certainly higher." 

Military bosses, in fear of being blamed for losses, could calculate that the best way to protect themselves is to depose Putin. Ordinary citizens, fed up with oppression, economic sanctionsEconomic measures designed to punish a country, for instance by excluding it from trade. and dying soldiers, could work together to overthrow their government. 

But Russian experts insist that neither option is likely. Vladimir Putin has spent the last 20 years preparing for the possibility of dissent - and taking steps to ensure that it is nearly impossible. 

Oligarchs know that they rely on Putin for their continued wealth. Russia's military is filled with spies; officials cannot trust each other. And in 2016, Putin created a separate domestic security service, the Russian National Guard, which is itself split into four branches. Any plot would require complex coordination.

Protesters face similar challenges. Their leaders are imprisoned or even killed. Independent media sites are shut down. 

And the support protesters enjoy is limited, despite sanctions. "Russians are famously stoical," points out journalist Owen Matthews. Independent polling shows that Putin's approval rating in Russia has actually risen, from 71% in February to 83% today. 

Most experts agree with Matthews that "Putin's mask of invincibility has cracked". The question of whether it will finally fall off entirely will affect the people of Russia - and even the whole world - for decades to come. 

Yes:  Every military loss in Ukraine makes the possibility of a revolt in Russia more likely. Generals will take action to prevent a costly war. And the Russian people will not put up with hardship for long.

No: Most academics agree that Putin's grip on power is too strong to fail. Military bosses rarely undertake coups without a guarantee of success. And anyway, there is no potential replacement to step in.

Or... It took 13 years for the Russo-Japanese war to topple Tsar Nicholas II. Brezhnev's invasion of Afghanistan came 12 years before the USSR's end. Putin may be wounded, but his fall will not be quick.