The Russian soldier was angry. Half of his brigade were injured or dead. So, during yet another dangerous battle outside Kyiv, he turned his tank around and ran over his commander. 

Vladimir Putin's war in Ukraine is not going according to plan. "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,"  said GCHQ director Jeremy Fleming in a rare speech. 

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 a revolt topple Putin's regime? 

Historians say there are two main ways authoritarians fall: military coups or popular uprisings. 

Military bosses, in fear of being blamed for losses, could calculate that the best way to protect themselves is to depose Putin. Ordinary people, fed up with oppression, economic sanctions and dying soldiers, could work together to overthrow their government. 

But Russia experts say that neither option is likely. Vladimir Putin has spent the last 20 years "coup-proofing" his government. 

Russia's military is filled with spies, so officials cannot trust each other. And the military and security services are split into multiple branches, so any plot would need complex coordination. 

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

Meanwhile, many Russians support the regime. Independent polling shows that Putin's approval rating in Russia has actually risen, from 71% in February to 83% today. 

One thing is clear: the future of Putin will affect the people of Russia and 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 Russian people will not put up with hardship for long.

No: Most academics agree - 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.