President Volodymyr Zelensky pushed back against doubts about Ukraine’s battlefield prospects and about the support of its allies, telling reporters on Tuesday that his country is not losing the war and is prepared to negotiate with Russia, but only on its own terms.

Ukraine’s military fell far short of its goals in a counteroffensive over the summer, but Mr. Zelensky emphasized that it has strengthened its air defenses and scored naval successes in the Black Sea, while Russia has little but heavy casualties to show for its own war effort this year.

“Russia failed to achieve any results,” Mr. Zelensky said at a news conference promoting the opening of talks with the European Union about eventual Ukrainian membership. The negotiations, announced last week, offer a rare glimmer of hope after months of diplomatic and military setbacks.

The shocks of the Russian invasion in February 2022, the initial setbacks and the subsequent victories of the first year of war produced a remarkable unity within Ukraine and among its backers. But the second year exacted enormous human and financial costs without much movement on the ground, leaving Ukraine in a state of uncertainty in battle, in relations with its allies and in internal politics.

Looming over the country’s fight with Russia are delayed decisions on military and financial aid from the United States and the European Union. The United States has provided about half of Ukraine’s weaponry and ammunition directly to its army and about a quarter of the foreign aid to the Ukrainian budget, while Europe has led in providing financial assistance.

But Republicans in Congress have held up President Biden’s request for $64 billion more in support for Ukraine, along with aid to Israel and Taiwan, saying they will not approve it without overhauls in policies on immigration and border security.

And a European Union decision on a $54 billion, multiyear financial assistance package for Ukraine was delayed until January as Hungary blocked what needed to be a unanimous decision by the 27-member bloc.

Mr. Zelensky said he expected both Europe and United States to deliver. “I think we will not be betrayed by our partners,” he said. “I am confident the United States will not let us down.”

He offered his most extensive comments to date on the implications for Ukraine sustaining its war against Russia if Donald J. Trump — who has taken a benign view of the Kremlin and an adverse one of Kyiv — were to win the U.S. presidential election next year.

Without naming Mr. Trump, Mr. Zelensky said that if a future American president would pursue policies “that will be colder, or more inward oriented, if they will economize more, then these signals would have a very significant impact on the course of the war.” He compared a potential pullback by the United States to a crucial part being removed from the machinery of global security.

“The mechanism starts breaking,” he said.

On Monday, the Biden administration said it would announce just one additional package of military aid under spending already authorized by Congress before funding runs dry.

“When that one is done,” the National Security Council spokesman, John Kirby, told reporters, “we will have no more replenishment authority available to us.” Some military aid could still flow from a separate program overseen by the Pentagon, however.

A sense of drift in what has been a long and grueling conflict is evident at home, too.

Mr. Zelensky said military commanders recommended that Ukraine draft 450,000 to 500,000 men next year — an enormous figure for a country of about 40 million people — to sustain the war effort and allow soldiers who have fought continuously for 22 months to rotate away from the front. The military’s general staff, he said, has not provided a definitive plan with precise numbers for a mobilization, which would have to overcome widespread draft dodging and growing hesitance to serve in lethal trench warfare.

The call-up, Mr. Zelensky said, will be “sensitive,” conceding this and other grave challenges Ukraine faces.

At times during his news conference, he betrayed flashes of anger, lashing out at one reporter who questioned why he had not formed a national unity government with opposition parties.

Each side has insisted that it was willing to negotiate a peace — but only on terms the other calls unacceptable. Russia demands recognition of Russian sovereignty over four Ukrainian provinces it now only partly controls, as well as over Crimea, which it seized in 2014.

Ukraine has been rallying international support for its own 10-point peace formula, and Mr. Zelensky reiterated on Tuesday that he is willing to negotiate on those terms. His government’s efforts are seen partly as an effort to parry Russian diplomatic outreach among developing nations and partly as positioning for talks. When the proposal is finalized and endorsed by the several dozen countries in talks now, it will be conveyed to Russia, Mr. Zelensky said.

He said Russia did not seem inclined to negotiate now. “We don’t see any request from Russia,” he said. “Not in their rhetoric, not in their action. We just see brazen willingness to kill.”

At home, tensions between Mr. Zelensky and the top military commander, Gen. Valery Zaluzhny, have spilled into public, stirring worries of a breach between Ukraine’s political and military leadership. The general is seen as a potential rival to Mr. Zelensky, though he has denied any political ambitions.

After The Economist published an interview last month in which General Zaluzhny said that fighting would be deadlocked along the front without new weaponry, Mr. Zelensky quickly contradicted that assessment of a war bogged down. The two have not been seen together in public since.

Mr. Zelensky did little Tuesday smooth over the rivalry. He said General Zaluzhny served as an appointee like others in his government. “I am thankful for some and ashamed of others,” he said, without clarifying his thoughts on the general.

The general has also publicly aired grievances; on Monday he criticized the president’s dismissal last summer of the heads of military recruitment offices over corruption allegations. The general said that the fired directors “were professionals — they knew how to do it, and now they are not here.”

Mr. Zelensky’s once sky-high approval ratings have slipped. A poll released this month by the Kyiv International Institute of Sociology showed 62 percent of Ukrainians trusted Mr. Zelensky, down from 84 percent last December.

Trust in General Zaluzhny, meanwhile, is at 88 percent, the poll found.

Ukraine’s leaders see some cause for hope in talks on joining the European Union, which would pull the country closer to economic and political integration with the West and out of Russia’s orbit.

The fight at sea has also brought success, even as the land war has stalled.

Deploying an innovative fleet of exploding sea drones and British- and French-provided cruise missiles, Ukraine has damaged or sunk dozens of Russian ships and forced the Black Sea Fleet to partially relocate from its home harbor of Sevastopol, in Crimea.

Firing Patriot air defense missiles, Ukraine has shot down some of the most sophisticated new weapons in Russia’s arsenal: hypersonic missiles the Russians call Kinzhals, or daggers, and which are known in Western militaries as Killjoys.

Ukraine, Mr. Zelensky said, will hold together in the fight against Russia. Unity, he said, “is our know-how, it helped preserve us since the beginning of the war.”

Invoking the country’s spirit, he said, might sound banal, but “one has to keep some banalities, just to have the ultimate banality of life.”