Vladimir Putin is placing a cynical bet that he can invade Ukraine just one week before a NATO summit - and that NATO will do nothing to stop him. The alliance must prove him wrong.

Despite sharp words from Brussels, Washington, London and Berlin, the Russian president believes that NATO lacks the will to challenge his dismemberment of Ukraine. By sending troops, tanks and artillery directly into the Ukrainian fighting, Putin is making a point: He will fight for Ukraine, and NATO will not. He is calling NATO's bluff.

The Western response will be read carefully from Kiev to Tallinn to Moscow. For the sake of Ukraine's integrity as a country, for future European security and for NATO's credibility as a defense organization, NATO leaders need to make some tough decisions and push back militarily against Russia.

NATO has already taken significant, positive military steps concerning its members in the east - particularly Poland, the Baltic states and Romania. This is important: The alliance's only obligation is to collective defense.

That must be sacrosanct. NATO has increased air policing over the Baltics, expanded exercises, promised to strengthen its defense planning and decided to deploy ground forces temporarily in Eastern Europe. These strong steps will cause Russia to think twice before expanding its aggression from Ukraine to NATO member states.

However, drawing such a bright line around NATO territory is being read by Putin as a signal that non-members such as Ukraine, Georgia and Moldova are - literally - up for grabs. With Russia's invasion of eastern Ukraine in the open, NATO needs to focus not only on defending alliance members but also on crisis management and projecting power beyond NATO territory.

To prove Putin wrong, NATO should take the following steps at its Wales summit:

? Provide direct military and intelligence support to the Ukrainian government. This means advisers, trainers, equipment and the possibility of direct reinforcement using NATO air and ground capabilities. Anti-tank weapons and air-defense systems should be in the mix. The most critical need is tactical: helping Ukraine use its own equipment and troops to reestablish a border with Russia, isolate separatists and avoid firefights to the greatest extent possible.

? Cancel all allied sales of military and dual-use equipment to Russia. The most notorious case is the French naval assault vessels - Mistral and Sebastopol - but other allies, including Britain and Germany, have yet to scrap all of their sales to Russia. As for the French ships: NATO should buy them for itself using its infrastructure budget and deploy them as a naval component to the NATO Response Force.

? Impose further sanctions - including on Gazprom and its leadership. Russia believes it has the upper hand in deterring strong Western action by threatening energy supplies. Europe needs to call Russia's bluff, showing that it can survive an energy showdown better than Russia can. The United States and Europe should also give a renewed push to European energy security efforts - including speeding delivery of U.S. liquefied natural gas (LNG) to Europe, and establishing a joint U.S.-E.U. financing mechanism to spur completion of LNG terminals and pipeline interconnectors.

? Establish a multinational NATO military presence on the territory of Poland and the Baltic states. NATO Secretary General Anders Fogh Rasmussen said this week that such action may be taken. It is critical that NATO prevent expansion of Russian aggression - and the best deterrence is preventive deployment. Any complaints that such a step violates the 1997 NATO-Russia Founding Act are negated by the first clause of NATO's undertaking: "In the current and foreseeable security environment." Russia's actions have fundamentally changed the security environment foreseen in 1997.

? Re-affirm NATO's commitment to building a Europe whole and free and at peace. Montenegro is ready for NATO membership now. Macedonia should be invited based on an urgent resolution of the dispute regarding its name. Finland and Sweden should be told they are welcome any time. And NATO should renew its pledge to work with Ukraine, Georgia and other partners on reforms necessary to help them qualify for membership. NATO must not accept a Russian diktat over the affairs of neighboring states.

Although it did not start out this way, the upcoming summit in Wales may be the most important NATO gathering since Prague in 2002, when NATO added seven members.

The signal the organization sends next week - whether it will stand up for European security or concede to Russian aggression - will ripple through Europe for years to come.

Kurt Volker is a former U.S. ambassador to NATO and executive director of the McCain Institute for International Leadership at Arizona State University. Erik Bratt-berg is a visiting fellow at the McCain Institute from the Swedish Institute of International Affairs.