Associated Press file photo / Ukrainian servicemen load an artillery shell before firing at Russian positions on the front line in Kharkiv region of Ukraine, on Wednesday.

When Russia’s full-scale invasion of Ukraine surpassed 1,418 days last month, it officially exceeded a historic milestone — the same span of time it took Moscow to defeat Nazi Germany in World War II.

And unlike the Red Army that pushed all the way to Berlin eight decades ago in what it called the Great Patriotic War, Russia’s 4-year-old, all-out invasion of its neighbor is still struggling to fully capture Ukraine’s eastern industrial heartland.

After Moscow failed to seize the capital of Kyiv and install a puppet government in February 2022, the conflict turned into trench warfare with tremendous cost. By some estimates, nearly 2 million soldiers are dead, wounded or missing on both sides in Europe’s most devastating conflict since World War II.

Russia has occupied about 20% of Ukrainian territory since illegally annexing Crimea in 2014, but its gains after the Feb. 24, 2022, invasion have been slow. NATO Secretary-General Mark Rutte this month likened Moscow’s advance to “the speed of a garden snail.”

Russian troops have moved only about 30 miles into the Donetsk region of eastern Ukraine in the past two years in a grinding battle for control of a few strongholds.

Despite the slow pace and high cost, President Vladimir Putin has maintained his maximalist demands in U.S.-mediated peace talks, saying Kyiv must pull its forces from the four Ukrainian regions that Moscow illegally annexed but never fully captured. He has repeatedly brandished his nuclear arsenal to prevent the West from boosting military support for Kyiv.

A war of attrition

Initially involving quick movements of large numbers of troops and tanks in Russia’s opening blitz and Ukraine’s counteroffensive in fall 2022, the fighting morphed into bloody positional warfare along the 750-mile front line.

The Washington-based Center for Strategic and International Studies estimated Russian military casualties at 1.2 million, including 325,000 killed. It put Ukrainian troop casualties at up to 600,000, including up to 140,000 killed.

“Russia has suffered the highest casualty rate of any major power in any war since World War II, and its military has performed poorly, with historically slow rates of advance and little new territory to show for its efforts over the last two years,” it said, noting Russian troops were advancing an average of 70 meters a day in two years to capture the transport hub of Pokrovsk.

For the first time in military history, drones are playing a decisive role, making it effectively impossible for either side to covertly mass significant numbers of troops.

Since early in the conflict, Ukraine has relied on drones to offset Moscow’s edge in firepower and stem its advances, but Russia has drastically expanded drone operations and introduced longer-range optical fiber-tethered drones to avoid electronic jamming.