The destruction of Malaysia Airlines Flight 17 over rebel-held territory of Ukraine may be the outrage that will spur closer cooperation between Europe and the United States in punishing Russia for its aggression in that area.
It also could eventually serve as a wake-up call to Russian President Vladimir Putin that he has overplayed his expansionist hand.
The Ukraine government on Friday published transcripts of what it said were conversations between Russian officers leading Ukrainian separatists, apparently acknowledging that separatist forces fired the missile that destroyed the passenger aircraft.
Simultaneously, the Pentagon said it believed the Russian-made missile was fired by the separatists.
Mr. Putin's cynical response to the tragedy should be a further spur to action against his government. On Thursday he blamed the incident on violence in the Ukraine, blithely ignoring the fact that he himself had stirred up the violence and is supplying leadership and weapons to the pro-Russian separatists.
According to the Kremlin, Mr. Putin told his cabinet, "This tragedy wouldn't have occurred if there were peace in that country, or in any case, if hostilities had not resumed in southeast Ukraine. And certainly, the government over whose territory it occurred is responsible for this terrible tragedy."
By happy coincidence, President Barack Obama had announced new sanctions on Russia the day before the Malaysia airliner was destroyed.
At the time Mr. Obama said, "We have repeatedly made clear that Russia must halt the flow of fighters and weapons across the border. We have to see concrete actions and not just words."
But President Putin reportedly considered the first round of sanctions as mere pinpricks and has apparently counted on Europe's dependency on Russian energy supplies to deter tougher measures. That's the same cynical mindset behind Mr. Putin's previous aggression this year in Crimea.
The new sanctions ordered by President Obama were designed to counter Mr. Putin's assumptions about the weakness of the sanctions on Russia.
These new restrictions target major banks and energy companies, the Russian defense industry, and individuals the White House said were responsible for the continuing support of the Ukrainian separatists.
The new sanctions - still stronger ones are on hold - went beyond what European governments have agreed to, but the Malaysia airlines tragedy appears to be changing European opinion.
"It can only accelerate the European move towards sanctions against Russia," a Central European diplomat told the Financial Times of London. "It will be easier to get agreement on additional measures."
The toughest sanctions hold a big risk for Europe because they could lead to the irreplaceable loss of Russian energy supplies, although the Russian economy desperately needs the revenue from those sales.
That means it is essential for the United States and the European Union to get together on a long-range energy policy that works to break Russia's stranglehold on European energy markets.
Also essential: Holding President Putin to account for the reckless land grabbing that led to Thursday's mass slaughter in the skies over Ukraine.