As Russian tanks roll into Ukraine in the first invasion by a world power against a sovereign European nation since World War II, the world is watching helplessly.

We should have seen this coming, not just in the past six months, but for the past 16 years. Russian President Vladimir Putin’s increasing level of aggression has long been apparent. In 2005, Putin stated that the collapse of the Soviet Union “was the greatest geopolitical catastrophe of the century.” In the years after, he has invaded and attacked one former Soviet republic after another: some with cyberweapons and others with brute force. These earlier invasions were a test to gauge reaction if Putin decided to take more offensive action.

It’s now clear that the United States and NATO underestimated Putin and his resolve. The pivot to the Pacific and particularly the U.S. recognition of China as its greatest threat has encouraged Putin. Seen from this perspective, the Ukrainian invasion is not the final step in Putin’s plan but may be the first. Putin’s long-held dream of reuniting nations of the former Soviet Union may be starting today, and in the process, irreparably damaging the credibility of U.S. commitments to allies and partners.

What form this will take is hard to tell. In its heyday, the Soviet Union consisted of 15 republics. Whatever comes next will not replicate the Soviet Union of old. Putin has different options, such as loose alliances or the integration of puppet republics. He might also only reintegrate those states that he claims are part of the “Russian world” or those he claims are “poorly governed” or already predisposed to ally with Russia. Belarus, Russia’s neighbor and Ukrainian invasion staging ground, is another likely target.

The bigger question is what will happen to the others. The Baltic states of Estonia, Latvia, and Lithuania, too, are highly dependent and geographically close. Even though they are member states of NATO, the alliance has not made a credible case for their defense. If Russia moves, will NATO defend? Russia has perhaps taken the gamble that it won’t.

As Putin escalates his aggression, the answer is becoming more clear. Loose alliances and tacit control are not enough. As he claimed in his speech earlier this month, Putin doesn’t believe in borders as the West sees them. He believes that “the disintegration of our united country was brought about by … historic, strategic mistakes.”

With this in mind, there are three steps the U.S. should take now to make Putin reconsider and stop his invasion.

First, the U.S. should lead the coalition of the willing, most importantly NATO, to bring in military aid for Ukraine, including artillery, large arms, and fuel as they defend their country. Second, the U.S. should cut off Russia’s sale of liquefied natural gas to Europe, a significant source of the Kremlin’s income. The U.S. and partner nations have been prepositioning additional LNG shipments, in a key strategic move that may now allow the West to pressure Russia. And lastly, the step Putin most fears, bring Ukraine into NATO.

Financial sanctions, words, and promises aren’t enough. They will not deter Putin because he was prepared and willing to accept the cost. These escalated responses will put in jeopardy that which Putin wants most — “political power and control at home” — the only thing likely to make him reconsider his actions. Putin is serious about reuniting the former Soviet Union. We must act now. The whole world is watching.

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