NATO needs to plan beyond efforts to deter another Russian invasion of Ukraine and recognize that its actual response may differ depending on the extent of Russian military operations.

Alliance foreign ministers met recently in Riga to consider how they should deal with Russia’s troop buildup near Ukraine and how to deter a repeat of the 2014 Russian occupation and annexation of Crimea, or worse. After the meeting, Secretary of State Antony Blinken told the press that NATO was united and would respond with a range of high-impact economic sanctions, as well as additional efforts to reinforce alliance defenses on the eastern flank.

In 2014, Putin was not deterred by economic sanctions. Should deterrence fail again, much of the actual American response may depend on where and how much of Ukraine Putin attempts to occupy.

Before acting, Putin will need to remember that the United States has a long history of reacting more forcefully to a major military provocation than its adversaries had expected. The Korean War and Desert Storm are just two prominent examples.

One might conceive of four broad possible scenarios for a Russian military invasion of Ukraine should Putin take such an unwise step. They are highlighted below from least to most likely. This analysis does not consider hybrid efforts such as instigating a coup d’etat as Ukraine’s President Volodymyr Zelensky fears. Western responses to a military attack would vary depending on which alternative Putin chooses.

The first alternative is to occupy all of Ukraine. This seems unlikely since it would be very risky diplomatically and expensive militarily for Moscow. Russia could lose. Putin appears interested primarily in Russian-speaking areas of eastern Ukraine. The Western response to an all-out invasion could be fierce, including possibly providing airpower to Ukraine to defend Kyiv should Ukraine be losing the battle. It could result in escalation and a major war despite lack of an Article 5 commitment. Putin likely knows this. Therefore, he is probably—hopefully—deterred here.

Putin’s second alternative is to seize all of Ukraine’s coastline from Donetsk to Moldova, including Odessa. If successful, Ukraine would be land-locked, it would lose up to half of its territory, and Putin would have control over the territory of most Russian speakers in Ukraine. But he would also have to occupy the primarily Ukrainian-speaking area between Odessa and Crimea, which could be very hard to hold onto militarily. The Western response would also be severe, including possibly some form of closer relationship with NATO for what is left of Ukraine in order to help protect it. This is not a likely option for Russia either unless Putin has strong initial military success and the West balks.

The third is to seize the coastline from Donetsk to Crimea along the Sea of Azov, through Mariupol. This is mostly Russian-speaking or bilingual territory. In addition, controlling all of the Sea of Azov would have great strategic value for Russia. Crimea would be connected by land to the rest of Russia. This too would result in very heavy fighting, and Russian troop losses would be significant. There would likely be a strong NATO reaction. However, the West would probably be less likely to send in military force to support the Ukrainian military directly since the activity is not near NATO’s borders and is primarily in Russian-speaking areas. Massive Western sanctions and arms shipments to Kyiv would be likely.

The fourth alternative is the annexation of the Donbas region currently occupied by Russian forces. This might be accomplished with or even without the deployment of additional Russian troops into Ukraine. Russia’s current force posture on Ukraine’s borders could serve as a message to Kyiv not to respond to annexation militarily. This would be the least dangerous of the four alternatives for Putin since Russian troops are already there and he has already gotten away with one annexation. Nonetheless, he would lose all pretense of his “little green men” excuse and of Russian innocence. The West would probably respond diplomatically, with much stronger economic sanctions, and with massive arms transfers to Ukraine, but not with Western military deployments to defend Ukraine.

This fourth alternative may be Putin’s most attractive option if he decides to use force. It’s an alternative for which NATO needs to plan.

The hope must be that Putin will take up Secretary Blinken’s invitation at Riga to reengage in the Minsk 2 peace process and not pursue one of these military alternatives. The world is already a very dangerous place. A Russian military attack on Ukraine would make it much more so.

Hans Binnendijk is a Distinguished Fellow at the Atlantic Council and served previously as Director of NDU’s Institute for National Strategic Studies and as NSC Senior Director for Defense Policy and Arms Control. Barry Pavel is Senior Vice President at the Atlantic Council and Director of the Scowcroft Center for Strategy and Security. He served previously as NSC Senior Director for Defense Policy and Strategy.

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