Avril Haines, the director of national intelligence, warned the Senate Armed Services Committee recently that the next few months may be volatile. The conflict between Ukraine and Russia could take “a more unpredictable and potentially escalatory trajectory,” she said, with the increased likelihood that Russia could threaten to use nuclear weapons.

These are extraordinary costs and serious dangers, and yet there are many questions that President Biden has yet to answer for the American public with regard to the continued involvement of the United States in this conflict.

In March, this board argued that the message from the United States and its allies to Ukrainians and Russians alike must be: No matter how long it takes, Ukraine will be free. Ukraine deserves support against Russia’s unprovoked aggression, and the United States must lead its NATO allies in demonstrating to Vladimir Putin that the Atlantic alliance is willing and able to resist his revanchist ambitions.

Is the United States, for example, trying to help bring an end to this conflict, through a settlement that would allow for a sovereign Ukraine and some kind of relationship between the United States and Russia? Or is the United States now trying to weaken Russia permanently? Has the administration’s goal shifted to destabilizing Vladimir Putin or having him removed? Does the United States intend to hold Mr. Putin accountable as a war criminal? Or is the goal to try to avoid a wider war — and if so, how does crowing about providing U.S. intelligence to kill Russians and sink one of their ships achieve this?

Without clarity on these questions, the White House not only risks losing Americans’ interest in supporting Ukrainians — who continue to suffer the loss of lives and livelihoods —but also jeopardizes long-term peace and security on the European continent.

Americans have been galvanized by Ukraine’s suffering, but popular support for a war far from U.S. shores will not continue indefinitely. Inflation is a much bigger issue for American voters than Ukraine, and the disruptions to global food and energy markets are likely to intensify.

The current moment is a messy one in this conflict, which may explain President Biden and his cabinet’s reluctance to put down clear goal posts. All the more reason, then, for Mr. Biden to make the case to American voters, well before November, that support for Ukraine means support for democratic values and the right of countries to defend themselves against aggression — while peace and security remain the ideal outcome in this war.

A decisive military victory for Ukraine over Russia, in which Ukraine regains all the territory Russia has seized since 2014, is not a realistic goal. Though Russia’s planning and fighting have been surprisingly sloppy, Russia remains too strong, and Mr. Putin has invested too much personal prestige in the invasion to back down.

The United States and NATO are already deeply involved, militarily and economically. Unrealistic expectations could draw them ever deeper into a costly, drawn-out war. Russia, however battered and inept, is still capable of inflicting untold destruction on Ukraine and is still a nuclear superpower with an aggrieved, volatile despot who has shown little inclination toward a negotiated settlement. Ukraine and Russia now “appear further apart than at any other point in the nearly three-month-long war,” as The Times reported.

Recent bellicose statements from Washington — President Biden’s assertion that Mr. Putin “cannot remain in power,” Defense Secretary Lloyd Austin’s comment that Russia must be “weakened” and the pledge by the House speaker, Nancy Pelosi, that the United States would support Ukraine “until victory is won” — may be rousing proclamations of support, but they do not bring negotiations any closer.

In the end, it is the Ukrainians who must make the hard decisions: They are the ones fighting, dying and losing their homes to Russian aggression, and it is they who must decide what an end to the war might look like. If the conflict does lead to real negotiations, it will be Ukrainian leaders who will have to make the painful territorial decisions that any compromise will demand.

The United States and NATO have demonstrated that they will support the Ukrainian fight with ample firepower and other means. And however the fighting ends, the U.S. and its allies must be prepared to help Ukraine rebuild.

But as the war continues, Mr. Biden should also make clear to President Volodymyr Zelensky and his people that there is a limit to how far the United States and NATO will confront Russia, and limits to the arms, money and political support they can muster. It is imperative that the Ukrainian government’s decisions be based on a realistic assessment of its means and how much more destruction Ukraine can sustain.