Yesterday afternoon, by most accounts, pro-Russian separatists shot down Malaysia Airlines flight 17 over eastern Ukraine. The attackers ostensibly thought that the Boeing 777 was a Ukrainian plane about to enter Russian airspace. Soon after the attack, Igor Girkin, the self-styled commander of the Donetsk People’s Army, bragged on his website that “We just shot down an AN-26 plane near Torez; it’s scattered somewhere around the Progress mine. We warned them not to fly in ‘our sky’.” Soon after, RIA Novosti, a Russian news agency, seconded Girkin’s claim.

After it became apparent that the plane was not Ukrainian, Girkin erased his post and Aleksandr Borodai, the prime minister of the self-proclaimed Donetsk People’s Republic, tried to put the blame for the attack, which killed 295, on Ukrainian authorities. Later in the day, Russian President Vladimir Putin stated that it was “unquestionable that the state over whose territory this took place is responsible for this terrible tragedy.”

The atrocity comes three days after Russian militants shot down a Ukrainian transport plane flying over Krasnodon district in Luhansk province and one day after a missile -- which Ukrainian authorities believe was fired by Russia -- brought down a Ukrainian SU-25 jet over Donetsk province.

This week also saw a major escalation of Russian military involvement in Ukraine; in the early morning hours of Sunday, July 13, about 100 Russian armored personnel carriers and other vehicles crossed from Russia into Luhansk province in Ukraine. Unlike earlier Russian deployments into Crimea and eastern Ukraine, these carriers were openly adorned with Russian insignia and flags. The flow of Russian tanks and soldiers into the area has since continued, and Ukrainian authorities estimate that up to 400 additional “little green men” (a term coined during the Crimea invasion for Russian troops without insignia) have infiltrated into eastern Ukraine’s Donbas.

Until yesterday, that escalation had gone relatively unremarked in Western media. But now, no matter who fired the missile, things are set to change. The downing of a civilian plane may conceivably qualify as a war crime, inasmuch as it entailed the unwarranted militarily destruction of a civilian target. At any rate, it was certainly an atrocity and an act of terrorism. And if Girkin -- an ethnic Russian who hails from Russia and who, by some accounts, is still an officer in the Russian military intelligence service, which would make him officially subordinate to Russia’s president -- really was involved, Putin might arguably be politically responsible for the crime.

Politically and economically, that couldn’t be worse news for Putin, who launched a charm offensive just last week at the World Cup in Rio de Janeiro. Putin, worried about the Ukrainian army’s rapid advances on insurgent positions, met with German Chancellor Angela Merkel and convinced her to agree to negotiations with the insurgents. His efforts -- presumably deemed insincere by Washington -- collapsed on Wednesday when the Obama administration imposed new financial sanctions on several important Russian banking and energy institutions, including Gazprombank, Novatek (an independent natural gas producer), the Rosneft Oil Company, and the VEB Bank for Development and Foreign Economic Affairs. Hours later, the Russian stock market took a nosedive and the ruble fell.

Putin might have managed to muddle along. Although most of the West has been deeply critical of Russia and its support for separatist groups in eastern Ukraine, European and American policymakers have been hesitant to impose the most severe sanctions and have seemed ready to move on to other foreign policy issues, such as Iraq and the war between Israel and Hamas. Even the Obama administration’s recent round of sanctions was not as far-reaching as many critics of the president would have liked.

But the Malaysia Airlines crash will force both the United States and Europe to come to terms with unpleasant realities. First, Russia has effectively embarked on a war against Ukraine. Kiev is no longer fighting homegrown insurgents and separatists; it is fighting Russian soldiers and Russian military equipment under Russian military command. War, unthinkable in Europe for so long, has truly come to the continent. Second, Russia also apparently believes that Donbas, the region over which the Malaysian flight was travelling, is Russian territory and that terror is a perfectly justified means for keeping control of it. As a new Amnesty International report has made clear, Russian forces have systematically engaged in human rights abuses against civilians in the eastern Ukrainian regions they rule.

In a word, even before yesterday, Donbas was well on its way to becoming Ukraine’s Bosnia -- with Putin playing the part of Serbian leader Slobodan Milosevic, Russia playing the part of Serbia, and Putin’s Donbas supporters playing the part of Bosnian Serb irregulars. Once Bosnia became a killing field, Europe and the United States could no longer turn a blind eye. NATO forces intervened in September 1995 with Operation Deliberate Force; two months later, the war ended with the signing of the Dayton Peace Agreement.

A direct Western military intervention in Ukraine remains unlikely. But other military assistance has now become possible for the simple reason that, if it did down the plane, Russia has already crossed the very red line that Washington had feared a more robust response in Ukraine would lead it to transgress. The United States, for its part, has ample military equipment in Iraq and Afghanistan, which could easily be diverted to Ukraine.

This week’s tragedy could remove any last shred of hope that Putin could be a valuable interlocutor in the Ukraine crisis. It is not impossible that he will realize that continued war with Ukraine is a lose-lose proposition and decide to use the crash as an opportunity to reinvent himself as a peacemaker who can pressure the separatists in Ukraine, hammer out some deal with Ukrainian President Petro Poroshenko, and declare victory. That doesn’t seem likely. But if he doesn’t, Russia’s cold war with the West could warm up considerably.