One million are now dead or injured in the Russia-Ukraine war

Determining the exact number of dead and wounded in the conflict has been difficult, with Russia and Ukraine declining to release official estimates or, at times, putting out figures that are widely mistrusted.

A confidential Ukrainian estimate from earlier this year put the number of dead Ukrainian troops at 80,000 and the wounded at 400,000, according to people familiar with the matter. Western intelligence estimates of Russian casualties vary, with some putting the number of dead as high as nearly 200,000 and wounded at around 400,000.

The losses are causing problems for Russia as it uses waves of poorly trained soldiers to try to advance in Ukraine’s east while also trying to counter a recent Ukrainian incursion in the Kursk region. But they are significantly more damaging for Ukraine, with a population less than one-quarter the size of its giant neighbor’s.

The high—and fast-rising—tolls on both sides highlight what will be a devastating long-term effect for countries that were struggling with population declines before the war mainly because of economic turmoil and social upheavals. They also illuminate one of Russian President Vladimir Putin’s own motivations behind launching the invasion in 2022: to boost Russia’s population by absorbing Ukrainians. Russia’s invasions and capture of Ukrainian territory over the past decade have caused Ukraine to lose at least 10 million people under occupation or as refugees, according to government estimates and demographers.

Putin has long declared addressing Russia’s chronic demographic decline a priority, and the Kremlin has since embarked on a campaign of Russifying occupied territories, including large-scale abduction of children and pressuring Ukrainians to obtain Russian citizenship. In the occupied Donbas region, selling property and other transactions now require obtaining Russian citizenship.

Today’s Ukraine was once part of the Russian Empire, and Putin has repeatedly said he seeks to revert the country to that state. He denies Ukrainian identity and statehood and claims that Ukrainians, a largely Slavic and Orthodox Christian people, are in fact part of the Russian nation.

“Demographics is a priority for Putin, and he wants to use Ukraine and its people to consolidate the Slavic core of Russia," said Ivan Krastev, a Bulgarian-born political scientist and an author of a coming book on European demographics. “But for Ukraine, the dilemma is existential: How many people can you lose in a war before losing your future?"

Putin’s single most effective measure to boost Russia’s population before the full-scale invasion was the annexation of Crimea from Ukraine in 2014, which added around 2.4 million people to Russia, according to Krastev, who based his estimates on the latest Russian census.

While Russia has gained population by grabbing territory, the war has had a devastating effect on its internal demographics and the labor market. Well over 600,000 Russians fled the country since the full-scale invasion started. They are mainly younger and upwardly mobile professionals who were able to afford relocating to foreign countries and starting a new life.

Russia has traditionally relied on labor migration from Central Asia, but the war reduced, and in some cases even reversed, the flow of migrant workers. This exacerbated the growing labor shortage in Russia as Siberia and the Far East are rapidly depopulating. Government-linked experts have publicly floated the idea of importing workers from North Korea.

Russia’s assaults on Ukraine have had a catastrophic effect on its neighbor’s population. The most recent census, in 2001, recorded 48 million inhabitants. At the start of 2022, before Russia invaded, that had fallen to 40 million, including regions such as Crimea that Russia occupied in 2014, according to Ukrainian demographers and government officials. With over six million fleeing Ukraine since the start of the war in February 2022, according to the United Nations, and Russia seizing further land, the total population on Kyiv-controlled territory has now dropped to between 25 million and 27 million, according to previously undisclosed Ukrainian government estimates.

Oleksandr Gladun, a researcher at the Ptoukha Institute for Demography, gave higher estimates of 42 million for the prewar population of all of Ukraine and around 29 million living on government-controlled territory at the start of this year. The population of Ukraine can only be calculated a couple of years after the end of the war when the number of returnees will be clear, he said.

The effect could be enduring. Alongside military deaths, Ukraine’s birthrate also collapsed to the lowest recorded level: In the first half of this year, three times as many people died as were born, according to government data. Some 250,000 deaths and over 87,000 births were recorded in this period, which is 9% less than the same period last year, according to government figures. In 2021, the year before the full-scale invasion, over 130,000 births were recorded.

Russia’s way of war is also aimed at making Ukraine unlivable. Russian missile-and-drone attacks have knocked out large parts of Ukraine’s energy grid, including power stations, which could drive many more Ukrainians to seek refuge outside the country this winter if it leads to major electricity and heat outages.

Ukraine’s government, like that of Russia, keeps its war casualties secret. President Volodymyr Zelensky said in February that around 31,000 soldiers have so far been killed. Several former political and security officials said that underestimate was largely designed to placate society and help continue the mobilization of much-needed new recruits. A spokesman for Zelensky declined to comment.

One of the key reasons Zelensky refuses to mobilize the key cohort of men aged between 18 and 25—typically the bulk of any fighting force—is because most of these people haven’t had children yet, according to the former Ukrainian officials. Should the recruits of that age group die or become incapacitated, future demographic prospects would dim further, Ukrainian demographers say.

Ukraine has therefore resisted calls from Western partners to throw more men into the fight and has only implemented partial mobilization. The average age of Ukrainian fighters is now over 43, according to estimates by government and military officials. Kyiv has been recruiting small numbers of convicts and foreigners to boost numbers.

The civilian death toll remains unknown. Russia’s 2022 conquest of the southeastern port city of Mariupol alone claimed over 8,000 lives, according to estimates by Human Rights Watch, a nongovernmental organization.

Ievgeniia Sivorka contributed to this article.

Write to Bojan Pancevski at bojan.pancevski@wsj.com