Russia's President Vladimir Putin. (Pool/Reuters)

EASTERN UKRAINE remains a violent caldron as Ukrainian soldiers shell pro-Russian separatists in the cities of Donetsk and Luhansk. It appears that Ukraine’s forces are making headway, while the separatists seem to have lost ground. There’s a sense that the crisis may be at a turning point. German Chancellor Angela Merkel is to visit Ukraine on Sunday and, on Aug. 26, Presidents Vladimir Putin of Russia and Petro Poroshenko of Ukraine will meet in Minsk, Belarus, to discuss the conflict.

With so many innocent civilians caught up in lethal combat, it is tempting to look for a cease-fire or some kind of time out that would lead to a period of diplomatic negotiation. But what would a pause and diplomacy accomplish? Any negotiations that leave this blight festering in Ukraine must be avoided. The only acceptable solution is for Mr. Putin’s aggression to be reversed.

Aggression is the right word. Although the separatists may not be wearing Russian military insignia, no one should be under any illusions: This was a rebellion with roots in Moscow. After seizing Crimea, Mr. Putin set a wildfire ablaze in eastern Ukraine in order to meddle and control. Mr. Putin’s approach has been terribly sly, from the “little green men” who took over Crimea without noticeable military insignia, to the “uprising” in eastern Ukraine of separatist fighters who just suddenly happened to possess anti-aircraft missiles. Mark Galeotti of New York University wrote recently in Foreign Policy that Mr. Putin has demonstrated in Ukraine a method of fighting with his military intelligence service that is “a mix of stealth, deniability, subversion, and surgical violence.” We would add: outrageous lies and propaganda.

The answer to these tactics is not to compromise and legitimatize them. Any discussion that leads to a shred of success for Mr. Putin’s nonlinear war would encourage the use of such tactics again. Mr. Putin must be shown that it does not work and that the West has the fortitude to block his subterfuge.

A second reason to push back is to deny Mr. Putin the benefits of an unresolved dispute. If Mr. Putin can keep the battle for Donetsk simmering, he can keep Ukraine off balance and under his thumb. It would hurt Ukraine’s chances for integration with Europe, which Mr. Putin wants to spoil. The Kremlin leader may also be calculating that the United States and its allies will lose interest and leave him free rein. No cease-fire or diplomatic bargaining should be contemplated that would effectively freeze this conflict in place, creating another Abkhazia or Trans-Dniester.

If conflict in the east is prolonged, even in a low-key fashion, it could poison Ukraine’s future and once again threaten its fragile stability. That would be a win for Mr. Putin and a loss for everyone else.