Both armies have tanks, artillery and thousands of soldiers ready to face off on the battlefields of Ukraine in a long-anticipated Ukrainian counteroffensive against Russia. But one thing clearly sets the two sides apart: time.

Ukraine is feeling immense short-term pressures from its western backers, as the US and its allies treat the counteroffensive as a critical test of whether the weapons, training and ammunition they have rushed to the country in recent months can translate into significant gains.

If the Ukrainians fall short of expectations, they risk an erosion of western support. It is a source of anxiety for top officials in Kyiv, who know that beyond battlefield muscle and ingenuity, victory may ultimately come down to a test of wills between the Kremlin and the West — and which side can muster more political, economic and industrial staying power, possibly for years.

As a result, there is a sense in Ukraine that its war effort faces a ticking clock.

“In countries that are our partners, our friends, the expectation of the counteroffensive is overestimated, overheated, I would say,” Ukraine’s defence minister, Oleksiy Reznikov, said in an interview this past week in Kyiv, the capital. “That is my main concern.” The expectations of military success are only one pressure point for Ukraine. A presidential election in the US looms next year, with the potential for a new, less supportive Republican administration.

President Vladimir V. Putin faces his own challenges but is showing signs of operating on a much longer timeline, encumbered by economic and military limitations but free from the domestic political pressures that make continuing western support for Ukraine so uncertain.

Having already mobilised some 300,000 recruits last September, Putin is laying the groundwork for a possible new round of conscription, having changed the law so Russian authorities can draft men by serving them with a “digital summons” online.

In private conversations, his defence minister, Sergei K. Shoigu, has professed a willingness to dig in for the long haul, vowing to carry out more mobilizations if necessary and emphasizing that Russia is capable of conscripting as many as 25 million fighting-age men, a senior European official said.

Russia’s economy is under increasing strain, and its defence sector, like the West’s, is struggling to provide enough matériel for the front. There are signs of simmering anxiety over the Ukrainian counteroffensive. On Friday, Yevgeny Prigozhin, the leader of the Wagner mercenary group, castigated Russian military leadership over a lack of ammunition and threatened to pull his forces from the fighting in Bakhmut within days.

But Putin has defined the war effort as a top priority and vital national interest, telling Russians in a New Year’s address that “we must only fight, only keep going” against western democracies intent on Russia’s destruction.

“Certainly I think there is a calculation in the Kremlin that Russia is more resilient than the West,” said Thomas E. Graham, a distinguished fellow at the Council on Foreign Relations.

New York Times News Service