Stephen Blank is a senior fellow at the American Foreign Policy Council.

The tragic downing of Malaysian Airlines Flight 17 on July 17 by Russian-armed troops provides another occasion for the West to use its superior power to bring Russia's war in Ukraine to an end. This assertion may sound surprising but nobody denies that the West, if it acts in unison, possesses more than enough power to force Russia to stop its war in Ukraine, withdraw its soldiers who shot down this plane, and move out all of its weapons. Yet the West has only employed a fraction of its power to date, driven as it is by commercial considerations and misplaced geopolitical fear of Russia's reaction. It is precisely this disunity that has allowed Vladimir Putin to keep raising the ante in Ukraine because it lets him indulge his belief that he can outlast any Western pressure. After all, major energy firms have signed big deals with Rusisa while this fighting was taking place, confirming his belief in the West's essential decadence and greed.

Moreover he has so identified himself with the nationalist passion in Russian politics that he himself has generated that to retreat now would undermine his domestic political position and acknowledge a stunning geopolitical defeat caused solely by his obstinacy. If the West does not exploit this opportunity to impose truly powerful sanctions, Mr. Putin will likely continue to raise the stakes in Ukraine and be drawn into a deeper and still more protracted aggression that would truly increase the possibility for a general war.

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In other words, because nothing until now has convinced Mr. Putin to stop and because he has hitherto seen his enemies as weak and divided, unless they impose such severe sanctions that make the message of Western resolve crystal clear, he is likely to keep plunging. If the West wants to deter a greater or wider war from breaking out it must now seize control of the so called ladder of escalation. By imposing severe sectoral sanctions on the key sectors of Russia's economy – energy, banking, and finance – it can send Mr. Putin a message that continuing this war risks a wider war that Russia can neither win nor sustain.

The French Revolutionary Louis St. Just once acidly observed that those who make revolution by half steps are only digging their own graves. This insight also applies to the deterrence that the West should have provided before this crisis and since it began until now. Instead, the timorous half-steps and warnings backed up by nothing but air have led Mr. Putin to conclude that he can stand the sanctions imposed to date since they will probably not last and in any case the West is divided.

Moreover he has convinced himself that he cannot let Ukraine be an independent westward-looking state, for that spells the end of his system at home. As a result he has put the security and stability of Russia itself and Europe at greater risk than anyone has done in years. Paradoxically, a strong Western response, along the lines being called for by President Barack Obama and Prime Minister David Cameron are essential to impose deterrence upon Russia, rescue it, Ukraine, and Europe from a wider war, and begin the task of pacifying post-war Ukraine. Another round of half-steps will only confirm St. Just's observation although the forum for that justification will be war not revolution.

Indeed, the pathetically divided and hesitant Western response until now has allowed Mr. Putin to widen the war and maintain the strategic initiative. The sight of stronger, richer states cowering before Mr. Putin is more than a little reminiscent of the appeasers of the 1930s who feared what Hitler or Mussolini might do if they acted forcefully to thwart their aggressions in their early stages. While Mr. Putin is not Hitler – although he evidently aspires to something like Mussolini's status – the same lesson holds today. Those who resist aggression by half-steps are only digging their own (and others') graves. Thus the nearly 300 victims of the Flight 17 demonstrate the costs of inaction, along with the brutality and corruption of the Russian forces, largely composed of Russian intelligence, paramilitary, military, and volunteer forces.

The West must also act because Mr. Putin has repeatedly shown that he will not accept responsibility for his actions. This should not have surprised anyone. As a veteran KGB officer he and his colleague have long ago internalized the notion that their all their crimes were actually committed by the victims while they were saving the state. To let this kind of behaviour go unpunished, not only risks a wider war, it also further corrupts both Russia's and Europe's public morality. Once again the West has the opportunity to deter a war, rescue the latest victims of Russian aggression from its grasp and continue its historic mission to civilize international politics. If we forfeit that chance by not imposing the deterrence, punishments, or sanctions clearly required here, who knows when, if ever, we will get a second chance to do the right thing.

Stephen Blank is co-author of an upcoming project with the Macdonald-Laurier Institute on Russia's ambitions in the Arctic.