IN BRUSSELS on Friday, Western leaders will meet to fashion a response to Russia’s continuing aggression in Ukraine.

Leading up to the conference, Russian President Vladimir Putin has tried to forestall a new round of sanctions with gestures suggesting de-escalation. But these have been just that — gestures that are relatively insignificant, easily reversible or both. Mr. Putin, for example, had the Russian parliament rescind its authorization to send troops into Ukraine, a measure it can easily and quickly reinstate if the Kremlin decides it needs any authorization later.

Meanwhile, Russia’s behavior remains unacceptably provocative. Russia continues to occupy Ukrainian territory in Crimea, it has not applied its influence to end the uprising it sponsored in eastern Ukraine and it continues to deploy forces to Ukraine’s border. Ukraine’s new president, Petro Poroshenko, reported that at least 18 Ukrainian soldiers have died this week, despite his declaration of a cease-fire. On Tuesday pro-Russia separatists shot down a Ukrainian helicopter, killing nine on board. According to Secretary of State John F. Kerry, the weapon used in the attack was Russian.

On Thursday Mr. Kerry declared that “it is critical for Russia to show in the next hours, literally, that they’re moving to help disarm the separatists, to encourage them to disarm, to call on them to lay down their weapons and to begin to become part of a legitimate process.” These are clear and appropriate “red lines.” The problem is that Mr. Putin already has crossed such lines, and too often the consequences have been weak or nonexistent.

Unfortunately, the signals from Western leaders heading into Friday’s meeting were hazy. European leaders have been the most hesitant, because their economic ties to Russia are thicker, but U.S. business leaders, too, have been lobbying in a counterproductive fashion.

Armed pro-Russia militants take a break as they storm the Ukrainian National Guard unit in the eastern Ukrainian city of Donetsk on June 26, 2014. (Alexander Khudoteply/AFP/Getty Images)

The West’s political leaders have to put principle above the special pleading. Ukraine lived peaceably as a sovereign state since becoming independent in 1991; the rebellion in the east is manufactured by Russia to undermine Ukraine’s sovereignty. The United States and Britain guaranteed support for that sovereignty in 1994 when Ukraine gave up its nuclear weapons. Now Ukraine’s government has offered generous terms to the rebels in the east. Mr. Poroshenko has promised and pushed forward reforms that would give the region more autonomy and protect the use of the Russian language. Though he has every right to secure his country’s borders and combat a radical insurgency within Ukraine, Mr. Poroshenko unilaterally established a cease-fire and offered amnesty to separatists.

If Russia continues to support rebels who reject these terms, the West must ramp up its support for the Ukrainian government. The United States is right to work for allied unity on sanctions. But the quest for unity cannot become an excuse for inaction. In the coming days — or even the coming “hours” — if Mr. Putin does not back down and Europe shrinks from acting, the United States must take the lead and then pull allies in the right direction.