Russian President Vladimir Putin waves to people in the city of Gorno-Altaisk, Siberia, Russia on Thursday, Sept. 4, 2014. (Alexei Nikolsky/AP)

IN CARRYING out his stealth invasion and lawless land grab in Ukraine, Russian President Vladimir Putin has tried hard to divide the Western alliance and make it look impotent. It’s critical that he not succeed.

On Monday, the European Union decided on tough sanctions against Russia to punish it for sponsoring and participating in the latest military aggression against its sovereign neighbor. The sanctions were to be announced in coordination with parallel moves by the United States.

But no sooner did the Europeans agree to the sanctions than they balked at their implementation. A handful of E.U. members begged for more time to allow a shaky cease-fire to take hold between pro-Russian separatists and the Ukrainian government.

That’s precisely the sort of weak-kneed response that Mr. Putin is hoping for. If a unified Western alliance cannot use the economic and commercial leverage at its disposal to help the government in Kiev dislodge Russian troops and weapons from eastern Ukraine, Moscow will have succeeded in establishing another “frozen conflict” and destabilizing another of its neighbors.

The status quo in eastern Ukraine should not be acceptable to the United States or any European nation. In the past few weeks, Russian troops and Russian-backed separatists have succeeded in occupying a new chunk of Ukrainian territory, forcing Kiev’s battered army to retreat. Moscow’s forces have threatened to overrun the Ukrainian Black Sea port of Mariupol , a major step toward establishing a land bridge from Russia to Crimea, which Moscow succeeded in annexing last spring.

Mr. Putin has paid an insufficient price internationally for invading an independent country. He has brazenly lied — to his own people as well as to the world — by denying that Russian troops are in action against fellow Slavs in Ukraine. In hopes of asserting a new Russian sphere of influence, he believes himself justified in dismembering Russia’s neighbor.

In agreeing to a cease-fire last week with Ukrainian President Petro Poroshenko, Mr. Putin signaled that he was willing to pull back troops and de-escalate tensions in return for autonomy for Russian-speaking parts of eastern Ukraine. But without the pressure of muscular Western sanctions, he is unlikely to uphold his end of the deal.

The sanctions prepared by Europe and Washington include measures to block key Russian oil companies from accessing Western capital markets. Other steps would block key Russian military suppliers from raising money in the West. Those are sanctions with teeth. They should be accompanied by meaningful military and economic assistance to Ukraine.

By standing fast, the West may eventually succeed in turning back or at least constraining Moscow’s neo-imperial ambitions. The sanctions could be relaxed, but only in return for evidence that Moscow was helping to implement a lasting political settlement that does not imperil Ukraine’s sovereignty. If the West cannot muster such unity, however, no one should be surprised when Mr. Putin’s next move is to press ahead with his invasion.