Russia expected a swift victory against Ukraine when it first invaded the country last February.
A combination of strategic mistakes and strong Ukrainian resistance dashed those hopes.
Russia has since improved its weapons and defense, setting the stage for a long, deadly war.
When Russia first invaded Ukraine in February of last year, Moscow shocked much of the West and the world when the country managed to fumble an apparent military advantage.
But strategic mistakes and unexpectedly strong resistance from Ukraine dashed President Vladimir Putin's hopes of a swift victory.
A senior US defense official acknowledged in February of last year that Russia has been frustrated with its performance. He said, however, Russia would reevaluate its strategy.
More than a year later, that premonition has come true: Russia shifted its defense tactics and changed the tides of the war using deep trenches, cheap drones, and a war machine producing more artillery shells than the West had anticipated, The Wall Street Journal reported.
"We have seen quite a few areas where they're adapting, and of course, we're paying close attention to that," Gen. James Hecker, the commander of US Air Forces in Europe, told The Journal.
The result of the country's shift was made clear this summer with the glacial pace of Ukraine's counteroffensive.
Ukrainian troops struggled to break through Russia's defensive lines, plagued with barbed wire, land mines, and anti-tank ditches.
George Barros, a Russia analyst at the Institute for the Study of War, previously told Insider the slow rollout of weapons support from the West provided Russia "time to reequip or reconstitute themselves."
Russia has also adapted to Ukraine's weapons, setting up air-defense systems that can shoot down missiles and drones and tools that can jam the other side's GPS signals.
While Russia took down Ukraine's drones, the country had stockpiled cheap drones of its own from China, the Journal reported.
Ukraine managed to break through Russia's defense by the end of August, two months after it launched its counteroffensive, Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, said.
Defense experts say Russia may have made a "very costly" mistake by focusing too much on its first line of defense, making the second line more vulnerable and easier to break.
An Institute for the Study of War report also stated that Ukrainian forces recaptured two villages near the front lines, causing a "severe degradation" of Russian troops.
But Moscow's war machine continues to churn, the Journal reported.
An unnamed Western defense official told the newspaper that Russia was initially expected to produce about one million artillery shells a year. The official said the West now believed Russia was expected to produce two million artillery shells in the next couple of years.
Putin had indicated he was bracing for a long war in Ukraine as the death toll racked up, Reuters reported.
By mid-August, US officials roughly estimated Russia's military casualties included 120,000 dead and up to 180,000 injured, The New York Times reported. Ukrainian deaths, in comparison, were estimated to be about 70,000, with 120,000 wounded.
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