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The tragic death of 298 people on board Malaysian Airlines flight 17 in a wheat field in eastern Ukraine marks a moment when words of disapproval and expressions of grief are not enough. This is a moment when action must follow the outrage and rhetorical condemnation.

I believe this is a defining event in history.

It is a defining event for Russia and its President Vladimir Putin. It is no secret that Putin has imperial ambitions, motivated by his pathological insecurities and a quest to restore lost glories. These are dangerous delusions that, if not confronted firmly, will come to threaten us all.

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It is a defining event for the United States and the European Union. The festering danger in Ukraine is a result of the civilized world’s faltering half-steps as a meager response to Russia’s invasion of a neighbor and violation of sovereign borders. This is an opportunity for American leadership, in step with our European allies, to spur the community of nations to act together and be a force for good.

And it’s a defining event for President Obama and German Chancellor Angela Merkel. These two leaders can stand up and demonstrate leadership that will shape history.

Given the significance of this moment, what are we to do?

1. Ask the entire civilized world to join the United States in condemning this outrageous act
Events like this tragedy have no place in the modern world, and this unassailable fact must be acknowledged globally. The condemnation needs to be so loud that the Kremlin will hear it.

2. Demand full and complete cooperation with the ongoing investigation
While many details about the crash are starting to emerge, we all want a full and detailed examination and accounting of exactly what happened. Our commitment to rule of law, rules of evidence, and the demands of justice require that we go through the investigative process, and we must insist on the access to do so.

We must demand full, immediate, unhindered access to the site of the tragedy – including all parts of the aircraft, missile battery, and site evidence – and most of all, proper treatment of the remains of the many victims. President Putin can himself ensure that access, and he absolutely must be required to do so.

3. Demand an immediate stand down in Ukraine
Crimes like Malaysia Airlines Flight 17 could only happen in such a lawless wasteland of renegades and desperados with their fingers on the triggers of the world’s most advanced weapons.

Lawlessness reigns in Eastern Ukraine because the government of that nation does not yet have sovereign control of its own territory.

This situation is a direct result of Putin’s outrageous territorial aggression that has already severed an arm of Ukraine and threatens the entire country’s disintegration. The Russian separatists in Eastern Ukraine have been organized, motivated, trained, equipped, unleashed, guided, and controlled by the forces of the Russian Federation, who are controlled with totalitarian thoroughness by Vladimir Putin.

Now we see a new tragic result of this aggression and sponsorship of ruthless renegades – a blatant act of terrorism inflicted on innocents. This problem will only get worse unless we demand that Russian behavior change and Putin’s aggression stop.

The only solution to the Ukraine problem, one that is consistent with international law, the demands of order and civility, and the requirements of justice, is that the government of Ukraine must have sovereign control over its own territory.

4. The U.S. and Europe must at last act vigorously and in unison
Until now, President Obama has sent largely weak signals to Putin about the seriousness of Russia’s actions. And our European partners have been reluctant to act, hypnotized by anxiety about their economic dependency. Let us hope after the horrific act of terror against 298 innocent passengers on Malaysia Air Flight 17, this is changing.

History will see this event as a watershed moment. Some argue that the Soviet downing of Korean flight 007 in 1983 was an event that exposed the true nature of the Soviet regime, and hastened its decay. Similarly, Malaysia Airlines Flight 17 reveals to any remaining doubters the nature of Putin and his brutal ambitions and ruthlessness.

With illusions stripped away, and the inadequacy of half measures revealed, we must now act together. We can respond to this tragedy by forging a new unity. Only the most robust and concerted actions to impose economic sanctions on Russia has a chance to change Putin’s behavior and end Russian support for the separatist militants.

We and the Europeans together must impose those costs. We need to target the fragile, dependent Russian economy through sanctions on Russia’s energy sector and state-backed arms exporter.

While it may take some time for Russia to feel the effects of sanctions on its energy sector, we can take action today that would have an immediate effect.

I have introduced legislation that prohibit all government contracts with Putin’s arms dealer.Taking steps to meaningfully obstruct this agency’s work and the revenue it provides the Russian state is among the most effective ways we can condemn Putin’s aggression.

Through these specific sanctions, we can demand that Putin end his support for the separatists and accept a stable Ukraine. If not, we must bring Russia’s economy to its knees.

This crisis has reached a point of high tension, great tragedy and escalating consequences. These potential consequences are dangerous for us all, but most of all, for Putin’s Russia.

Russia’s president holds in his hands the ability to deescalate the crisis or pay a very steep price.

It is Putin’s choice to bring this situation back from the brink. It is our obligation – along with our European partners – to make his choice crystal clear.