A heated and sometimes nasty debate is raging over how to end the war in Ukraine, intensified by the less-than-hoped-for advances of the Ukrainian counteroffensive and shifting American support for continuing aid to the country. A CNN survey released on Aug. 4 found that among liberals, 69 percent back more funding, but only 31 percent among conservatives do.

Proponents of seeking a cease-fire say an outright military victory by either side is highly unlikely, and they argue that the sooner the carnage and destruction end, the sooner Ukraine can start building the sort of democratic, prosperous state that would amount to a moral victory over Russia.

The hawks argue that a cease-fire would effectively reward Russia’s aggression with substantial territorial gains and that only a decisive military victory over Russia can prevent Vladimir Putin or his successors from future land grabs. The debate has been complicated by Donald Trump’s claim that if elected, he would end the war “within 24 hours” and by opposition to continuing aid for Ukraine among other Republican presidential candidates. That gives Russia a strong incentive not to negotiate until after the 2024 election.

Any talk of a cease-fire or negotiations is premature; neither side is ready to negotiate, and the United States is in no position to compel them to come to the table. Mr. Putin, who chose to wage this war, could end it today simply by calling back his troops. Yet he continues his claim to nearly a fifth of Ukraine’s territory and has shown no interest in a cease-fire. He has gambled from the outset that time will erode the Western commitment to Ukraine, and he would most likely read any push toward a cease-fire as confirmation that his strategy is working.

And so long as Russia is not ready to talk, neither can Volodymyr Zelensky, the Ukrainian president, whose publicly declared goal is to drive the Russians out of all Ukrainian territory, including Crimea and the Donbas. Ceding any territory to the Russian invader is anathema to Mr. Zelensky and the Ukrainians, and they would not agree to a cease-fire that might be used by Mr. Putin to regroup and attack again.

The moment when both sides conclude that they have nothing more to gain on the battlefield, most likely the prerequisite for negotiations even to be considered by either side, still appears a long way off.

President Biden’s position from the outset of the war, shared by America’s allies, has been to give Ukraine the weapons and resources to defend itself, so that when the time comes, it would be “in the strongest possible position at the negotiating table.” But the decision on negotiating belongs to Ukraine, as Mr. Biden wrote in a guest essay in The Times in May and has reiterated many times: “I will not pressure the Ukrainian government — in private or public — to make any territorial concessions.”

While this board has questioned some specific decisions by Mr. Biden, such as supplying the Ukrainian Army with cluster munitions, we agree with him that it would be “wrong and contrary to well-settled principles” to pressure another country to negotiate over its sovereign territory.

Ukraine deserves full support against Russia’s unprovoked invasion, and it is in America’s national interest to lead its NATO allies in demonstrating that they will not tolerate Mr. Putin’s revanchist ambitions. It is a demonstration of America’s commitment to democracy and leadership that other would-be aggressors are watching.

Firmly backing Ukraine, however, does not preclude an open debate on the scale and extent of America’s support, or on how the war might end. On the contrary, a commitment of this magnitude and consequence requires debate to justify public support, especially in today’s sharply rived politics.

The divide will come to a test soon when a $40 billion supplemental funding request, with $24 billion earmarked for Ukraine, comes up before Congress, and it is certain to become more pronounced as the presidential race quickens.

The Biden administration has already set some constraints around its support for Ukraine in this conflict, and it’s necessary to continuously calibrate support against those limits. The administration has rightly insisted, for example, that it will not assist Ukraine in attacking Moscow or in any other way that could draw NATO into a direct war with Russia.

But there are other questions to wrestle with: Would a massive rout of Russian forces drive Russia to deploy tactical nuclear weapons? What would be the ramifications of allowing NATO allies to supply Ukraine with F-16 fighter jets? While the F-16s would greatly enhance Ukraine’s ability to counter Russian missile attacks, the highly sophisticated planes represent a qualitative escalation of Ukraine’s military reach, so NATO will have to carefully consider which of the jet’s lethal capabilities to include in the deal.

Mr. Biden and his advisers should also be more candid with the American public about what they don’t know about the duration of this conflict and its outcome. Wars are unpredictable, and this one has taken many surprising turns, from the Russian invasion itself to Ukraine’s success in rebuffing Russia’s initial drive. Most recently, the mutiny by the Russian warlord Yevgeny Prigozhin and his subsequent death in a plane crash exposed the cracks in Mr. Putin’s military and the ruthlessness he needs to maintain his reign of fear. Now there are reports that Russia is preparing a new offensive of its own.

It is also incumbent upon the administration not to let Ukraine waver in its commitment to democracy and in the fight against corruption, especially in its military institutions and industries. Ukrainian anti-corruption groups and journalists and other civil society groups have done brave and essential work in holding their own leaders to account, and these institutions deserve continued American support.

Open discussion about the war and its progress need not indicate weakness or a wavering will to Mr. Putin. That is something President Biden and leaders of both parties — including Senator Mitch McConnell and Representative Kevin McCarthy, the Republican leaders in the Congress — can make clear to the Russian leader.

“It’s certainly not the time to go wobbly,” Mr. McConnell said recently, urging his colleagues to support continued aid for Ukraine. “Now, with Ukraine bravely defending its sovereignty and eroding Russia’s capacity to threaten NATO, it is not the time to ease up.” He and Mr. McCarthy can continue to press that message with their colleagues in Congress no matter what is issuing from their fringe. As Nikki Haley reminded Vivek Ramaswamy in the first Republican presidential debate when he spoke out against any further aid to Kyiv, Ukraine is a “pro-American country that was invaded by a thug.”

Mr. Biden’s immediate challenge is to sustain support, not only from Congress and the American public but also from allies.

The leaders of the Group of 7 — Britain, Canada, France, Germany, Italy, Japan and the United States — issued a strong statement in July affirming their “unwavering commitment to the strategic objective of a free, independent, democratic and sovereign Ukraine” that would be “capable of defending itself and deterring future aggression.”

Each country said it would begin negotiations with Ukraine on specific, long-term security arrangements. These commitments are essential to preventing swings in public opinion or changes in government from causing abrupt shifts in support from Ukraine’s major Western allies.

Discussions between the U.S. State Department and Ukraine over long-term security commitments began in early August, with a focus “on ensuring Ukraine has a sustainable force capable of defending Ukraine now and deterring Russian aggression in the future.” The two countries are drafting a “memorandum of understanding” that will be especially important in view of the growing politicization of aid to Ukraine and the growing hostility to it on the right. Drafting an agreement should be a priority for the administration, and Congress should be kept closely informed of its progress.

In the end, there is no way to tell how long it will take for peace talks to reach the horizon. America’s duty is to help ensure that Ukraine reaches that point strong and free, and that support can be sustained only by an open discussion and an informed public.