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Destroyed Russian tanks in the Sumy region, Ukraine, on March 7.IRINA RYBAKOVA/PRESS SERVICE OF/Reuters

Fen Hampson is Chancellor’s Professor at Carleton University’s Norman Paterson School of International Affairs. Janice Gross Stein is the Belzberg Professor of Conflict Management in the University of Toronto’s Department of Political Science and was the founding director of the Munk School of Global Affairs & Public Policy.

Are we edging toward a major confrontation with Russia over Ukraine that could lead to war among states armed with nuclear weapons? Not yet, but leaders in Washington and Europe are increasingly worried about the risks of escalation. They are taking steps to reduce the danger – but are they doing enough to put the brakes on escalation?

U.S. President Joe Biden set firm limits on U.S. military involvement long before the Russian invasion of Ukraine began. He made it absolutely clear that the United States will not become a co-combatant in a direct war with nuclear-armed Russia. He said early and often that no U.S. forces would be sent to Ukraine because it is not a member of NATO. He has not allowed U.S. intelligence-gathering aircraft to fly over Ukraine and he has consistently refused President Volodymyr Zelensky’s desperate pleas that NATO enforce a no-fly zone over Ukraine.

Preventing a conventional war with Russia is an obvious imperative because Russia has the largest nuclear arsenal in the world. In a not-very-subtle signal, President Vladimir Putin went to the command centre of Russia’s strategic forces to observe the firing of a Russian missile in the days before Russia launched its attack; he went on to raise the alert status of Russian strategic nuclear forces twice. His Foreign Minister, Sergei Lavrov, has also warned that any war with NATO would be nuclear. The use of nuclear weapons is often described as “unthinkable,” but Russia’s leaders are at the very least thinking about it.

So far there is no evidence that Russian nuclear forces have taken steps to increase their readiness, and skeptics insist that Mr. Putin and his colleagues are bluffing in order to deter NATO. But Russia’s military doctrine sets a lower threshold for the use of tactical weapons than does NATO, in part because Russia’s forces are vastly outnumbered by NATO’s collective military manpower and firepower. Mr. Putin knows that he is outgunned and is frustrated that the invasion of Ukraine is proving to be a much tougher battle than he was led to believe because of the extraordinary bravery of Ukrainian fighters and the massive airlift of supplies by the United States and its allies to Ukraine. If Mr. Putin is backed into a corner, it is possible that he may resort to using tactical nuclear weapons on the battlefield.

This would break through a long-standing taboo against the use of nuclear weapons any time, anywhere, for any reason. And no Western leader is willing to take the risk of calling Mr. Putin’s bluff that a war with Russia would be nuclear because no one has any confidence any longer that they know what he might do next.

The risk of escalation has set a ceiling on the kind of military support that the United States and European leaders are willing to provide to an embattled Ukraine. But pressures to break these barriers are growing as the Russian attack continues and as outrage deepens at the Russian military’s tactics. Its bombing and killing of innocent civilians and the attacks on Ukrainian refugees are leading to increased demands from outraged publics that governments act. Mr. Zelensky’s bravery and skillful leadership have been extraordinary, and he has mobilized support worldwide. Mr. Zelensky also recently called out the failure of NATO’s leaders to establish the no-fly zone that would help to protect Ukrainian civilians or transfer fighter-jets that would let the Ukrainian air force take on Russian fighters in the skies over Ukraine: “All the people who will die from this day will die because of you, as well.”

Dangerous days are ahead. As the siege of Kyiv deepens, and Russia’s ruthless bombardment of civilians continues, the pressure to do more will certainly increase. But doing more runs up against the risk of NATO becoming a combatant in an all-out war with a nuclear-armed Russia. “The window for doing easy stuff to help the Ukrainians has closed,” said Major-General Michael S. Repass. The longer this war goes on, the more dangerous it becomes – for Ukraine, for Russia and for NATO.

As difficult as this is to say and as difficult as it is to hear, thoughtful leaders should be looking for an exit strategy that limits the damage for everyone – even though what makes exit strategies so hard to find is that they always involve political compromises.

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