The rockets that killed dozens of civilians in the Ukrainian port city of Mariupol last weekend also blew up the futile Western strategy for stopping Russian aggression in Ukraine.

President Barack Obama and his European allies desperately hoped that sanctions would squeeze Vladimir Putin into accepting a negotiated solution to the conflict. They were dreaming that he would press Russian-backed separatists in Ukraine to abide by the so-called Minsk cease-fire agreement signed in September by representatives of Ukraine, Russia and Europe.

Instead, Putin’s proxies, who fired the rockets into Mariupol, have ignored the Minsk accord and are on the warpath to seize more Ukrainian territory. They are doing this, as Obama said Sunday, with “Russian backing, Russian equipment, Russian financing, Russian training and Russian troops.”

The assault on Mariupol could be the first stage in a Russian-backed effort to seize a land bridge to Crimea, which Russia has already annexed. “So far (the attack) is a testing exercise,” one European businessman told me by phone from Kiev, “an effort to test the reaction of the international community to see how much Europeans and the United States will push back.”

Time to respond

The time for that pushback is now.

It’s past time to provide Ukraine with defensive weapons to prevent further Russian aggression. In December, Congress passed the Ukraine Freedom Support Act, which authorized the president to provide such weapons. The White House, and European nations, have been reluctant to consider lethal aid lest Putin escalate further or derail efforts to implement the Minsk cease-fire.

But withholding defensive weapons has not kept Moscow from sending hundreds of artillery pieces, tanks, armored personnel carriers and missiles to Ukraine. “We’ve had a policy of not providing arms to Ukraine and we’ve seen Putin escalate a half-dozen times,” says former U.S. ambassador to Ukraine John Herbst. Contrary to the arguments against defensive weapons, he says, “The absence of Western support has led to escalation.”

In other words, not bolstering Ukraine’s defenses encourages Putin to seek the seizure of more Ukrainian land.

Herbst and several other top U.S. diplomats and military officers, including former Deputy Secretary of State Strobe Talbott and former NATO Supreme Allied Commander Adm. James Stavridis, will release a report next week outlining what kinds of military aid they believe the United States and NATO should be providing to Ukraine. They want Washington to enhance Ukraine’s weak defensive capacity against Russian long-range rockets with counter-battery radar and communication equipment, along with sending missiles to defend against Russian armored vehicles and tanks.

Obama still appears reluctant to send such equipment. But the Ukraine violence won’t stop until Putin is convinced it’s too costly to ignore diplomacy in favor of war.

Diplomatic solution unlikely

Had Putin wanted a diplomatic solution, the Minsk accord provided most of what Moscow had asked for: local self-government in the areas of eastern Ukraine held by Putin’s proxies, Russian language rights for the residents, and a Ukrainian government plan to fund economic development of the region. Had the accord been implemented, Ukraine no doubt would have dropped any pretense of seeking NATO membership; NATO members don’t seek Kiev’s membership and Ukraine only raised the issue when confronted with Moscow’s aggression.

Instead, Putin’s proxies virtually ignored the agreement. Sanctions alone haven’t stopped the Russian leader, who is ready to sacrifice his country’s economic well-being in his quest to control Ukraine. He has deflected public attention from the country’s economic woes by claiming they are due to a Western conspiracy to destroy Russia (and muzzling any attempts by media to say otherwise).

But there is good reason to believe that providing defensive aid to Ukraine could force Putin to reconsider his course.

“Putin has a serious liability in conducting this war,” says Herbst, because he has lied to his people about the direct involvement of Russian soldiers in the fighting — and about the casualties they have taken. Polls show Russians don’t want their sons fighting in Ukraine (they believe Putin’s claims that the war is being waged only by Russian-speaking Ukrainians). The burials of the increasing numbers of Russian soldiers killed in the fighting are kept secret.

When associations of soldiers’ mothers have tried to publicize the deaths they have been told they must register as foreign agents. Russian journalists who try to report on the casualties have been beaten.

Aid would pressure Putin

If the weak Ukrainian army were strengthened — soon — the cost of Russian aggression would be heightened. It would be impossible for Putin to hide the truth about the human toll of the war, and his popularity could slide.

Any delivery of weapons should be accompanied by a clear message to Putin that the West prefers negotiations and seeks implementation of the Minsk accord. Putin should be offered an off-ramp — if he is interested.

But the Russian leader won’t consider diplomacy unless the West makes the cost of his Ukraine venture much higher. “What we are seeing is not a Ukraine crisis but a Putin crisis,” says Herbst. “Putin has said he has a right to protect all Russian speakers. He does not accept post-Cold War boundaries. He’s clearly playing a larger game — and if he succeeds in Ukraine we should expect larger troubles elsewhere.”