Petro Poroshenko is president of Ukraine.

When 298 innocent people were shot out of the sky by a Russian missile 10 days ago, people everywhere finally began to understand what is at stake in Ukraine.

Half a year ago, I was not even considering becoming president of Ukraine. But like a great many Ukrainians at the time, I was disturbed that then-president Viktor Yanukovych constrained Ukraine’s future by rejecting an association agreement with the European Union, choosing a customs union with Moscow instead. Like so many of my countrymen, I believed that for Ukraine to become a modern and successful country, it needed to expand its ties with the West and end widespread corruption and abuses of power. Then, the authorities unleashed a murderous assault on demonstrators in Kiev, and Yanukovych and his partners fled to Russia, leading to Moscow’s decision to annex Crimea and support the violent separatists operating in eastern Ukraine.

Russia’s behavior has only worsened since I took office in June. Over the past several weeks, Ukraine has resisted Russian aggression and continues the fight against the Moscow-backed separatists. Russia has tested us with its transfer of cash, weapons and other equipment to the separatists and its vast anti-Western, anti-Kiev propaganda campaign, but we will not yield to its interference in Ukraine’s sovereignty or to the violence perpetuated by terrorists.

Moscow is playing a dangerously irresponsible game. My fears of the violence spiraling out of control were confirmed July 17, when the separatists used Russian missiles to shoot down Malaysia Airlines Flight 17. This attack on innocents should never have happened. Like giving a handgun to a child, Moscow permitted the transfer of a highly sophisticated surface-to-air missile system to terrorists. With such a large number of lives lost — many of them children — this was a major tragedy. Adding to the horror has been the casual desecration of the crash site by the separatists, who have blocked access by investigators, tampered with evidence, looted belongings and treated the dead like offal.

As families began to mourn, world leaders and investigators grappled with the unfolding investigation. Here, we look back at the reaction to the crash of MH17. (Tom LeGro/The Washington Post)

As president, I have laid the groundwork for peace talks with the separatists by calling for dialogue along with guarantees of the rights of all Ukrainians. But these calls have been rejected, and Moscow continues to fan the conflict. It has always been my goal to bring together the parties and work out solutions for all Ukrainians regardless of geography, ethnicity or language.

To help achieve these solutions, the West should begin thinking about a larger response to what has happened. As always, the United States should take the lead. Working together with the European Union, Washington can shape a worldwide coalition of nations in support of Ukraine to ensure that these terrorists are not able to strike again.

It is important that the United States has instituted stronger sanctions in response to Russia’s aggression in my country. However, in light of this tragedy, the United States should consider imposing even tougher and wider-reaching sectoral sanctions on Moscow. And Europe needs to follow the lead of the United States and impose sectoral sanctions of its own.

Russia needs to know that the international community is serious. It is long past time for it to act.

In addition to broader sanctions, my country needs expedited deliveries of assistance to help us police our borders and unwind the insurgency with minimal violence.

Even as we address the immediate threat to our country, Ukraine must attend to other pressing needs. Ukraine can use U.S. leadership and assistance in our efforts to curtail corruption, revive our financial institutions, reform our energy policy and liberalize our agricultural sector. Additionally, we need U.S. natural gas to shore up our energy supplies so that we cannot be blackmailed by Moscow. We need a reliable partner and ally to help fuel our nation.

Now is the time for the international community to stand with Ukraine. To stop Russia, it will take the global community acting in concert. My country is doing everything it can to take charge of our destiny: We are working hard to curb corruption, implement much-needed reforms and, above all, restore the Ukrainian people’s faith in their government. Working together, we will not allow Moscow to stand in the way of creating a new open, united and prosperous Ukraine.