In February 2014, Russia invaded Ukraine. Its military commanders seemed to overlook a unique but very effective weapon in Ukraine’s arsenal: the famous muddy season.

Russia invaded Ukraine on February 24, and it coincided with the so-called “muddy road season” or “Rasputitsa in Russian. It’s a phenomenon that takes place twice a year, first in spring — when the winter freeze subsides and the country’s terrain and unpaved roads become virtually unpassable as they turn to mud — and then in the fall, when there can be heavy rain.

Experts believe the mud contributed to Russia’s slowing of advance in some parts, particularly in the north. Online videos and images have circulated showing Russian tanks and trucks abandoned and unable to move on the muddy roads and fields of Ukraine.

This has prompted disbelief from Russia analysts and military specialists, who stated that Russia’s military commanders had to be better prepared for the terrain and capable of avoiding the chaos caused by Ukraine’s muddy spring terrain.

It’s a phenomenon familiar in the history books: Napoleon Bonaparte’s invasion of Russia in 1812 was famously slowed by the mud, as were Hitler’s armies, which invaded the then-Soviet Union in 1941 and encountered the same logistical problems posed by the mud and inhospitable terrain that Russian troops have faced in the last few weeks.

Experts said that Russia’s military ought to have known what the conditions would be for its forces.

It is also known as “Ukrainian water” or “rasputitsa”, in Russian. The time after winter, when you have impassable roads… This is something that has been around for centuries. Napoleon experienced this issue. It is an advantage for Ukrainians. This is why it was especially important in the northern part of Ukraine, where the terrain is more dense.” Maximilian Hess from the Foreign Policy Research Institute told CNBC.

Initial expectations were that Russia would win the war in Ukraine quickly. However, the Ukrainian army resisted and was able to arm Western allies with weapons.

Russia had already amassed more than 100,000 troops at its Ukrainian border and conducted military drills alongside its ally Belarus. Belarus lies to its north. Moscow repeatedly insisted that they did not intend to invade.

Despite the military exercises ahead of the invasion, military analysts have said the first phase of the war — which has seen Russia gain ground in the south and east of the country but fail to make strides in the north, with its forces now pulled back and concentrating on eastern Ukraine — showed a lack of planning, preparedness and tactical skill among its military command and soldiers, many of whom are conscripts.

He said that Russia’s inability deal with Ukraine’s muddy seasons “shows serious problems with professionalism of the military.”

It raises serious questions about me… what have the Russians been doing? [military]For almost 10 years, they have been practising and conducting drills in foreign invasion. But, the problem was that the units didn’t think enough or had the coordination necessary to get the right units to the right locations and then move in the most effective way possible to handle something. [the mud]For over 300 years, this has been a known problem.”

U.S. intelligence suggested that Russia had wished to invade Ukraine earlier in the yearHowever, China had ordered them to postpone their offensive so that it wouldn’t overshadow Beijing Winter Olympics which ended Feb. 20.

Sam Cranny-Evans, a research analyst at the U.K. defense think tank RUSI, told CNBC that most of Russia’s military vehicles would have been able to cope with the mud in Ukraine, but problems had arisen from multiple vehicles using the same tracks, a foreseeable problem for any military commander with a basic understanding of “terramechanics” — or “the interaction of soil with off road vehicles.”

He said that many of his vehicles could be used to move through mud, provided they did not drive on the same tracks repeatedly.

“But, I’d argue that some things have limited them more in terms of reliance on railroadheads and roads to their logistics,” he stated. He also said that Ukraine’s large size presented an additional challenge to Russia’s war machine, especially for units located farther from Russia like those in the north Ukraine.

Many of these units are now focusing on the eastern and southern areas, which is where the second phase in the conflict is taking place.