By the end of this week, America and Europe will have to make the most crucial and far-reaching decisions about the West’s relationship with Russia since the fall of the Berlin Wall. There can be no avoiding them, because any such avoidance would be a decision to drift further into appeasement, weakness, and an eventual conflict with Putin’s Russia on less favorable terms. The shooting down of a civilian Malaysian airliner by “pro-Russian separatists” in eastern Ukraine, with the loss of 298 lives, is both a catalyst and a turning point. Whether the turn is toward a more brutal war in Ukraine and a protracted cold war between Russia and the West or toward second thoughts by Moscow and a diplomatic de-escalation of the crisis, however, will depend on what the West now does.

That means the West must first decide on what not to do. It should not focus on holding an international/judicial inquiry into the shooting down of the airliner — as the Dutch, the Malaysians, and the families of the victims understandably want to do. The reason is simple and adequate: There won’t be a judicial inquiry unless the Russian government thinks it can fake evidence convincingly enough to cast doubt on the undoubted truth that it is responsible for what happened. The only inquiry therefore will be a “fixed” inquiry — and that is literally worse than useless. Russian and pro-Russian agents on the spot in eastern Ukraine are already removing evidence (including the black boxes), violating the corpses of the victims, and threatening legitimate observers. It is too late to rescue a real inquiry.

But no inquiry is necessary to discover the truth that is already known to the main players in slightly varying degrees. Russian government officials know exactly what happened all the way down to who actually fired the missile that destroyed the Malaysian airliner, whether they were serving Russian military intelligence officers, or recent Russian ex-military “volunteers,” or local neo-fascist “pro-Russian separatists,” or mercenaries from Chechnya, Central Asia, etc. Western intelligence officials know almost all of these things from satellite photographs, communications intercepts (thank you, NSA and “Five Eyes”), and their agents on the ground. Western publics and media know all of this in general but not always in particular — as do those Russians who access Western media through the Internet. The only people in the dark are those ordinary Russians who trust their own mass media, which provide them with thrillingly absurd anti-Russian conspiracies by the West.

The truth is as follows: The Russian state is responsible for shooting down a civilian airliner. All that is in doubt is its precise degree of responsibility. If the missile was fired by Russian officers or by others controlled by or in immediate contact with Russian military or intelligence, then Moscow’s responsibility is total. If the missile was given by Moscow to simpatico terrorists or mercenaries or volunteers, then its responsibility is only slightly less, while being also tainted by utter recklessness. This is the behavior of, at best, a low-level criminal state with an ethic of ruthlessness and a tradition of incompetence.

What, however, should be done with this truth? Western governments have two options. They can reveal this information in graphic detail at the United Nations, as part of a strong Western response — including further economic sanctions and the supply of advanced Western arms to Ukraine — to Russia’s war against Ukraine. Or they can privately threaten to do all these things in tough diplomatic negotiations with Russia designed to get a reversal of Putin’s aggressive and illegal policies. Our view is that the second policy will be credible only if we first implement the first. A policy of threatening terrible things but doing mild ones will be seen by the Kremlin as something it can sit out and wait to evaporate. The West must show that it is determined to defeat Putin while being also willing to work with a post-imperial Russia. To judge from the statements of Western leaders over the weekend, that now seems a realistic prospect. The atmosphere surrounding the Ukrainian crisis has changed. Until yesterday, most people, including most Western governments, thought that Putin would win — at least in eastern Ukraine; now they are less sure of that, and keener to see him lose.

Western success will come more quickly, however, if it rests on Western unity. Hillary Clinton’s argument that the main responsibility for handling the Ukraine crisis rested on the Europeans was a transparent attempt to avoid blame for the failure of her “reset” policy toward Russia (and to stay on good terms with the Obama administration in case it is tempted to cast that blame on her.) But it is also false historically. Only one European country — namely, Britain — joined the U.S. in signing the 1994 Budapest Memorandum, which guaranteed Ukraine’s sovereignty in return for the surrender of its nuclear weapons. Mrs. Clinton should recall this, since it was one of the signal achievements of her husband’s first administration in its campaign against nuclear proliferation. It has not become a dead letter simply to suit the needs of her presidential campaign.

European Union leaders — notably Angela Merkel, David Cameron, and François Hollande — who last week were wary of the stronger U.S. sanctions proposed by the Obama administration will meet again on Wednesday in a very different atmosphere. They will be receptive to still-stronger policies. But what, exactly? It would be a logical response to Moscow’s breach of the Budapest Memorandum to return tactical battlefield nuclear weapons to Ukraine. That would render any threat of a full-scale Russian invasion of Ukraine impossible and undermine Putin in domestic politics — it would then be he who had brought nuclear weapons to the very borders of Russia. (The fact that these would not be NATO nuclear weapons would not reassure Russians.) Of course, such a bold move is unlikely because of anti-proliferation policies, because it would complicate Ukraine’s attempt to recover Donetsk, because it would alarm Ukraine’s other neighbors, and — most important of all — because it would reduce the West’s control of the crisis in a major way. Still, it should be raised and discussed as a possibility, to underline the risks that ignoring treaties and international law invites. In the absence of such a single stroke of policy, the West should ensure that Ukraine receives as many conventional armaments as it needs to recover eastern Ukraine and resist any further Russian incursions. Reviving the earlier plans for anti-missile defense sites in Poland and the Czech Republic and moving more NATO troops to Poland and front-line NATO states should also be on the table. And these strategic measures should be backed by tougher economic sanctions that — as several European leaders have conceded this weekend — should not ignore Germany’s energy imports from Russia, France’s sale of warships to Russia, and Russian money in the London markets.

Almost certainly Europe and the U.S. will agree on sanctions and military measures that fall somewhat short of these proposals. But they will also exceed what anyone thought likely a month ago. How might Putin respond? He is a bold risk-taker. His instinct in these circumstances will be to play for time and, if he intends to continue his military subversion of Ukraine, to conceal this as best he can. But the West should not overestimate him.

Ever since the Ukraine crisis began last year when Russia closed its borders to Ukraine’s agricultural products in order to force Ukraine to join the proposed Eurasian Union and to abandon its (and President Yanukovych’s) aim of associate membership in the European Union, Putin has consistently wrong-footed himself. His economic blackmail persuaded Yanukovych to break off negotiations with the EU. But this announcement led to mass popular protests in Kyiv. When Moscow pressured Yanukovych into repressing these protests brutally, his government fell and was replaced by one that favored the EU relationship. When Putin annexed Crimea in response, he expected that the West would complain briefly and then return to business as usual. In fact, the Western alliance has imposed a moderate but escalating series of sanctions on Russia. When Putin’s propaganda apparatus insisted that the Kyiv authorities were dominated by dangerous neo-fascist parties, the Ukrainian elections showed those parties scoring 1 percent of the vote. When Putin encouraged the Russian-speaking people of eastern Ukraine to rebel against Kyiv, they not only failed to rise on cue but his aggressive actions even unified Ukraine against Russia, fostering a stronger Ukrainian patriotism. Last year he had control of all Ukraine by proxy; ten days ago he had control of Crimea and Donetsk; and now he has demonstrated the brutal state reality of his Potemkin revolution to a dozen or more countries by shooting down a civilian airliner containing their citizens.

It would be better for everyone if Russia were to use the shooting down of the airliner as a humanitarian excuse to cry halt, retreat from Ukraine, and allow the Ukrainian army to reestablish order and state sovereignty there. Everyone except Vladimir Putin, that is. Strongmen don’t do retreats well. But there must now be many more Russians, including some in the Kremlin, who, in the light of the burning airliner, wonder if Putin is worth it. The West should strengthen their hand by raising the cost of him to more-painful levels.