Certainly the United States and Europe should increase their aid to Ukraine and explore ways to expand existing sanctions against Russia. NATO’s commander, Gen. Philip Breedlove, is said to support providing weapons and equipment to Kiev. And Secretary of State John Kerry is said to be open to discussing the idea. But lethal assistance could open a dangerous new chapter in the struggle — a chapter Mr. Putin would quite possibly welcome, as it would “confirm” his propaganda claims of Western aggression.

So far, President Obama has cautiously pledged to help Ukraine in every way “short of military confrontation.” Yet with sanctions and diplomacy making no headway against Russian aggression, it is imperative that the United States and its allies take a new look at what would bring Russia to a serious negotiation.

The first question is, to negotiate what? Along with denying the direct involvement of his troops in eastern Ukraine, Mr. Putin has not made clear what he is trying to achieve. Russian officials have suggested that Moscow has no interest in annexing eastern Ukraine, the way it grabbed Crimea, but rather seeks a Ukrainian federation in which the pro-Russian provinces would have relative autonomy, along with assurances that Ukraine will not move to join NATO.

There is definitely potential for negotiations there. Yet the latest rebel attacks have focused on Mariupol, an important port on the Black Sea, and on expanding the rebels’ control to areas that would give their self-proclaimed “republics” greater military and economic cohesion. And that speaks to long-term rebel occupation.

Tempting as it is to focus on punishing Mr. Putin, the greater objective must be to end the fighting so that Ukraine can finally undertake the arduous task of reforming and reviving its economy. Toward that end, the West must make clear to Mr. Putin that if a federation is his goal, the United States and its allies will actively use their good offices with Kiev to seek a workable arrangement.