All week, a line of Russian tanks 40 miles long has been heading straight for Kyiv. When it arrives, it could strike the Ukrainian capital with extreme power.

Yet the Russian advance has slowed to a stop. Putin's forces are now an easy target.

Why? Food shortages. Not enough fuel. And too much mud. Tank tires need to be in very good condition to mow through soft surfaces. Left parked, they can begin to leak. This causes damage to the tyres.

Putin's tanks have not been well-kept. And there is currently a lot of mud. The region is just entering the spring Rasputitsa. This is a period of time where rain and melting winter ice lead to muddy roads. The Russian tanks are forced into a narrow row along paved roads. This makes them sitting ducks. 

Tanks and mud have long been enemies. The first tanks were used by the British Army in World War One. But many of them were abandoned after getting stuck in the mud. 

Difficult terrain has proved a challenge for armies throughout history - not just for tanks. When Napoleon attempted to invade Russia in 1812, around 180,000 members of his Grande Armee died as a result of poor weather conditions. 

This time, the mud could also interfere with Ukrainian efforts. The battlefield bogs of World War One took lives on both sides. 

It is also unlikely the mud will hold Putin up for long. The Soviet Union initially faltered in the Winter War for similar reasons. They adapted their tactics and quickly won.

Yes: If the world sees the Russian Army being picked off while stuck in the mud, Putin will be humiliated. It might force him to abandon his invasion in face of resistance.

No: Putin's convoy is just one part of his mighty war machine. The best mud can do is slow things down.

Or...? Mud is fickle. It can slow down the Russians, but it can also make it difficult for Ukraine to get around it. And if the invasion continues long enough, the soil will eventually harden.