As Russia Advances, U.S. Fears Ukraine Has Entered a Grim Phase

American military and intelligence officials have concluded that the war in Ukraine is no longer a stalemate as Russia makes steady gains, and the sense of pessimism in Kyiv and Washington is deepening.

The dip in morale and questions about whether American support will continue pose their own threat to Ukraine’s war effort. Ukraine is losing territory in the east, and its forces inside Russia have been partially pushed back.

The Ukrainian military is struggling to recruit soldiers and equip new units. The number of its soldiers killed in action, about 57,000, is half of Russia’s losses but still significant for the much smaller country.

Russia’s shortages of soldiers and supplies have also grown worse, Western officials and other experts said. And its gains in the war have come at great cost.

If U.S. support for Ukraine remains strong until next summer, Kyiv could have an opportunity to take advantage of Russia’s weaknesses and expected shortfalls in soldiers and tanks, American officials say.

U.S. government analysts concluded this summer that Russia was unlikely to make significant gains in Ukraine in the coming months, as its poorly trained forces struggled to break through Ukrainian defenses. But that assessment proved wrong.

Russian troops have advanced in the Donbas region of eastern Ukraine. They have clawed back more than a third of the territory that Ukrainian forces seized in a surprise offensive in the Kursk region of western Russia this year. The number of Russian drone strikes across Ukraine has increased from 350 in July to 750 in August and 1,500 in September.

“The situation is tense,” said a Ukrainian major stationed on the Ukrainian side of the border near Kursk who goes by the call sign Grizzly. “We are constantly losing previously occupied positions, the enemy has an advantage in men and artillery, and we are trying to hold the line.”Gone is the Russian force that repeatedly stumbled as it invaded Ukraine in 2022. The Russian military, according to a senior U.S. military official, has evolved and is “on the march.”

As a result, some American intelligence agencies and military officials are pessimistic about Ukraine’s ability to stop Russian advances as Kyiv tries to find ways to build up forces exhausted by nearly three years of war.

Still, Russia has fallen short of its own goals. Most notably, it has not been able to take the city of Pokrovsk, a logistics hub for Ukrainian forces. And independent experts say Russia’s shortages of radar, armored vehicles and, most critically, troops will come to a head next year.

The most important immediate development for Ukraine, however, will not be on the battlefield but at the ballot box in the United States. Former President Donald J. Trump and Vice President Kamala Harris have laid out very different visions for future American support.

Mr. Trump has promised to bring the war to a quick end, and his running mate, Senator JD Vance, has outlined a peace plan that looks a lot like one advanced by President Vladimir V. Putin of Russia.

Ms. Harris, on the other hand, has vowed to fight on, warning that if Russia was not stopped in Ukraine, its forces could attack NATO.

The election, and its uncertain outcome, is weighing heavily on Ukrainians.

After a meeting with President Volodymyr Zelensky in Kyiv last week, American officials said the Ukrainian leader looked worn and stressed, anxious about his troops’ battlefield setbacks as well as the U.S. elections.