A protester against Russia's president Vladimir Putin stands outside the Russian consulate in Kharkiv, Ukraine, on Monday. (Sergey Bobok/AFP/Getty Images)

THE CRISIS of the week may be in Iraq, not Ukraine. But Russian President Vladimir Putin has not lessened the pressure on Russia’s weaker neighbor. The West, which applied modest sanctions in April that seemed to affect Mr. Putin’s calculations for a time, must prepare clear and certain consequences if Russia’s provocations continue.

Over the weekend, NATO released satellite images indicating that Russian tanks have entered Ukraine in recent days, backing up reports that Russia has been sending heavy weaponry and vehicles into Ukraine’s east. On the same day, Russian-backed separatist rebels downed a Ukrainian military airplane, killing 49 people. On Monday, Gazprom, the Russian energy giant, suspended gas flows into Ukraine, after Russia rejected a reasonable offer brokered by the European Union. “This is not about gas,” Arseniy Yatsenyuk, Ukraine’s prime minister, said. “This is a general plan for the destruction of Ukraine.”

Petro Poroshenko, Ukraine’s new president, is familiar with these kinds of tactics. A billionaire chocolate manufacturer, Mr. Poroshenko saw Russia bar imports of his candy during a recent bout of political conflict. Now Mr. Poroshenko is supervising a military effort to reassert control over Ukraine’s restive east, the scene of a Russia-backed rebellion. He is also poised to sign a landmark trade pact with the European Union on June 27. His predecessor, Viktor Yanukovych, declined the E.U. association agreement; that set off the protests that led to his ouster, Ukraine’s movement away from Mr. Putin and Russia’s annexation of Crimea. The association agreement is a long way from E.U. membership, but it would nevertheless be a significant step as the country attempts to build a functional state capable of charting a middle course between east and west — if that is the course the Ukrainian people choose.

In that effort, Mr. Poroshenko is also preparing a peace plan meant to reincorporate Ukraine’s east on terms that will be acceptable to its many ethnic Russians, offering a cease fire, respecting the use of the Russian language, holding early parliamentary elections and granting amnesty to many of those involved in the recent pro-Russian agitation.

Mr. Putin may prefer that Ukraine remain divided, backward and dependent on Russian largess, too unstable to be an attractive partner for Western Europe. He is occupying Ukrainian territory, namely Crimea, and provoking civil conflict in a region that had lived peaceably inside Ukraine since the country gained its independence 23 years ago. If Mr. Poroshenko’s generous peace effort does not elicit a fair response, the United States and its European allies must be ready with a strategy and a timetable to respond decisively.