Russia’s aggression in Crimea, a replay of its 2008 invasion of Georgia, disrupts a post-Cold War order that has assured the sovereignty and independence of Russia’s neighbors. Now all of them--even NATO members in Central and Eastern Europe--feel less secure. Only determined Western action can deter future coercion by Russia, and raise the costs of militarily occupying Crimea.

As Secretary of State John Kerry has pointed out, Russia is violating international commitments. It’s not just the United Nations Charter. The 1990 Paris Charter for a New Europe pledged the USSR and other parties to “refrain from the threat or use of force against the territorial integrity or political independence of any state.” Russia has assumed this obligation. The 1994 Budapest Memorandum on Security Assurances, an agreement concluded in connection with Ukraine’s accession to the Treaty on the Non-Proliferation of Nuclear Weapons, committed Russia, the United Kingdom, and the United States to “refrain from the threat or use of force against the territorial integrity or political independence of Ukraine.”

Notwithstanding these pledges, President Vladimir Putin and his parliament assert Russia's right to intervene militarily to “protect” the interests of Russians in Ukraine. Recently Russian foreign minister Sergei Lavrov claimed Russian troops would stay in Crimea until the political situation "normalized" – the disingenuous justification the Soviets used in 1968 in Czechoslovakia.

Putin may think his occupation of Crimea will keep Ukraine from going westward and will advance Russia’s broader plans for a Eurasian Union aimed at restoring dominion over neighbors. Putin likely expects a Western rhetorical riposte over Russia’s actions in Crimea, but, as after the invasion of Georgia, not much more. Disappointingly, Russian public opinion backed the invasion of Georgia. Today, Putin may calculate that the seizure of Crimea will also boost his popularity.

Misunderstanding the West’s strength and his own country’s weakness, Putin seeks to convince Russians that the West lacks courage and conspires to harm Russia, that history justifies Russian control of Crimea, and that Russia’s more unified and authoritarian governance is superior.

So after Estonia, Latvia, and Lithuania joined the European Union and NATO in 2004, Putin seems to have drawn a line in the sand. Russia now occupies parts of three neighbors--Georgia, Moldova, and Ukraine--which have cooperative ties with NATO. Moscow has pushed other neighbors, such as Armenia, into joining the Russia-dominated, relatively protectionist Eurasian Customs Union. Notwithstanding the pressure, Georgia, Moldova and Ukraine remain eager to move closer to the EU.

Putin is abusing the international right to protect endangered populations when a sovereign state is incapable of doing so by asserting Moscow’s right to protect ethnic Russians in all former Soviet countries. The Kremlin is propounding false claims that the physical security of ethnic Russians in Crimea and eastern Ukraine is now at risk. This is not the first time. Russia has harshly criticized Latvia and Estonia regarding the status of local Russians, who seem eager to remain where they are.

Moscow’s objectives in Ukraine beyond Crimea are unclear, but hateful propaganda suggests the Kremlin is considering moving troops into eastern Ukraine, taking advantage of a new and untried national government in Kyiv. Putin would like to see it replaced and have its successor recognize Russia's interests in Crimea.

A united and determined Western response can make a difference. In 2008, Russia was on the verge of capturing the Georgian capital, Tbilisi, but the EU and America intervened with forceful diplomacy and public condemnation. Nonetheless, Russia's military assault discouraged NATO from taking in Georgia as a member, and resulted in greater Russian military presence in Abkhazia and South Ossetia, which Moscow declared to be “independent.”

Putin’s hand is not as strong as it looks. The Russian economy has slowed and is overly dependent on oil and gas exports, mainly to Europe. It is in a position to reduce dependence on Russian gas, benefiting from the current worldwide glut. Western technology and investment are essential for Russia to develop new and challenging oil and gas fields. The seething Islamist insurgency in the North Caucasus continues to bleed Moscow.

Much now depends on how the West reacts to Russia’s intervention in Crimea. The stakes are high. The West’s two-decade-old pursuit of a Europe “whole, free, and in peace” is on the line.

Some responses are taking shape. German Chancellor Merkel has proposed to Putin the positioning of international observers in Crimea to assess the Kremlin’s claims that ethnic Russians are under threat of violence under the new Ukrainian government. Stronger Western measures, however, are needed.

On March 2, Secretary of State John Kerry foreshadowed several: expulsion of Russia from the G8 group of leading industrialized democracies, and visa bans and asset freezes for officials. The EU as well is likely to undertake “targeted measures.” US and EU sanctions should apply to all those tied to Russia’s leadership and security forces.

The Obama administration ought also to announce expedited licensing for the export of liquefied natural gas to Europe, allowing it to reduce dependence on imports of Russian gas.

In his visit to Kyiv on Tuesday, Kerry should offer urgent US help to Ukraine. It needs substantial aid to avert bankruptcy, especially since it is mobilizing military forces and calling up reservists. The West ought to extend the help right away, and offer military advice and defensive supplies. NATO should share with Ukraine intelligence on nearby Russian military activities.

Looking beyond Ukraine, the West should seek to deter Russian coercion of smaller neighbors. While putting Western troops on the ground would be unwise, NATO countries ought to beef up warship patrols in the Black Sea, and intensify military exercises and patrols with allies in Central and Eastern Europe. Former NATO commander Admiral James Stavridis properly advises that NATO develop contingency plans to react to a full-scale invasion of Ukraine.

Russian intervention in Crimea is the most acute challenge to European security since the 1999 Kosovo war. Now another threat to European security looms, and the hour for Western action has arrived.

Denis Corboy is Visiting Senior Research Fellow King's College London, and served as European Commission ambassador to Armenia and Georgia. William Courtney was U.S. ambassador to Kazakhstan and Georgia, and special assistant to the President for Russia, Ukraine, and Eurasia. Kenneth Yalowitz is a Wilson Center Global Fellow and was U.S. ambassador to Belarus and Georgia.