Mr. Putin may seek to create Novorossiya one slender slice at a time, thereby reducing his chances of massive confrontation with the West. An intermediate Kremlin goal would be to connect Crimea by land to Russia. Mariupol stands in the way. Ukrainian volunteers of all ages are digging trenches there to block what they believe will be Mr. Putin’s next move. The West must decide how to prevent Mariupol from becoming another bloody road bump on Mr. Putin’s westward drive.

American and European leaders urgently need to construct a viable deterrent to Mr. Putin’s plans for Novorossiya. This would not include NATO boots on the ground, since few in government or the public at large in Europe and America support that option. But any plan to contain Russia must be tough to be effective.

Mr. Putin has said he wants the cease-fire to hold. The West needs to unite and force him to keep that promise. It needs to construct a Mariupol test, which will turn that town from a road bump into a red line with teeth.

Washington and its European allies need to decide now what they will do should the separatist fighters and their Russian enablers who took Donetsk and Luhansk appear in Mariupol in force.

Under such circumstances, Kiev must be given lethal weapons and training, not just by the United States but by the Europeans as well. Tougher economic sanctions should also be imposed. The West should publicly discuss suspending Russia from the Brussels-based Swift financial-messaging system, a step which could cripple the already reeling Russian economy. Strict new limits on visas for Russian travel to the West should also be imposed. Such measures could ignite a dramatic reaction from Russia, such as a natural gas cut-off to Europe. But absent the will to introduce Western ground forces, it will take stern measures such as these to deter Moscow.

Should the cease-fire hold and Ukraine regain full control of its territory, then the West may start to ease sanctions. Mr. Putin will need incentives for him to back down. That can be done without accepting what the Kremlin has gained through violence.

A carrot-and-stick approach also should have some appeal in Germany. If Germany leads on this issue the rest of Europe is likely to follow. As Ms. Merkel has said, the stakes for Europe are high. If military force can be used to change borders, the rules that have created a fairly unified, peaceful and prosperous Europe are open to revision everywhere.

Drawing the line at Mariupol in the manner we propose could constrain Mr. Putin enough to preserve the rule of law on the Continent. President Obama and Chancellor Merkel need to construct a deterrent proposal soon to make the “Mariupol test” a success.