The war of attrition between Russia and Ukraine is killing soldiers at a pace unseen in Europe since World War II.

Ukrainian artillery fire, explosive drones and mines are killing Russian troops, as they repeatedly charge across the no-man’s land. As Ukrainian positions are exposed, they are suffering heavy casualties inflicted from afar by Russian drones, shells and glide bombs.

Calculating the scale of the casualties, and therefore the war’s trajectory, is difficult: The information is a state secret in both countries. The Ukrainian government has been especially secretive, restricting access to demographic data that could be used to estimate its losses.

The most complete counts of Ukraine’s dead soldiers are made by groups abroad with biased or opaque motivations.

Working with incomplete information, these groups and other experts estimate that Ukraine has suffered about half of Russia’s irreplaceable losses — deaths and injuries that take soldiers out of battle indefinitely — in the nearly three-year-old war.