Nato leaders are meeting for an emergency summit in Brussels today to address the war in Ukraine. While there, they must discuss and agree on how to respond in the event of further escalation from Russia. In the best case, clarifying Nato’s response to further escalation will deter Vladimir Putin from taking that step. At the very least, it will prepare the allies for what may follow. Having failed to quickly oust the elected government in Kyiv, Putin now faces a choice between negotiating an end to the war, or escalating further. While talks are ongoing, Putin himself has given no indication that he’ll settle for anything less than Ukraine’s complete subjugation. Two weeks ago, CIA director William Burns told Congress that the Russian leader is “likely to double down”. He will “try to grind down the Ukrainian military with no regard for civilian casualties”. Putin could ramp up the war in many ways – from a deliberate decision to widen the war against Nato, to an attack on allied territory from a stray missile, to using chemical or biological agents or even a nuclear weapon. How should Nato respond to any of these scenarios? A deliberate attack on a member of Nato is unlikely because the allies have made clear they would defend “every inch” of Nato territory. An attack on Nato would mean war – and given the state of its armed forces, it would be a war Russia would be likely to lose. The other forms of escalation would require a more calibrated response. The possibility of a missile hitting Nato territory was underscored when Russia attacked a target just miles from the Polish border. What if the missile had landed in Poland instead? Nato could counter such escalation by taking out the launcher or aircraft that sent the missile. Though risky, such a proportionate response would leave the onus to escalate further on Putin. The use of chemical or biological agents would be a different story. US officials have repeatedly warned that false Russian accusations about Ukrainian chemical weapons and bioweapons labs may constitute a pretext for Moscow to use these weapons and blame Ukraine. How Nato responds should depend on the circumstances. Russia could bomb an industrial chemical or biological research facility and blame the resulting damage on Ukraine. It has already targeted an ammonia storage facility. Other such attacks could cause hundreds or even thousands of casualties, depending on the location, weather, and other conditions. Nato could not let such an escalation go unanswered. At the very least, it should take out the forces responsible for bombing the facilities with surgical strikes. The US, Europe, and other countries would also need to tighten economic sanctions – for example, banning imports of all Russian goods or denying all Russian banks access to the Swift banking system. Moscow, no doubt, would blame any release of chemical or biological agents on Ukraine, and could use those false claims to escalate further. This could include using industrial chemical agents such as chlorine or ammonia to target civilians, as happened repeatedly in Syria, with planes and helicopters dumping barrels full of chemicals on civilians below. Nato would have to respond to such barbarism, for example, by imposing a real no-fly zone to prevent further Russian aerial attacks. Of course, deploying Nato aircraft into a war zone would require attacking Russian aircraft and air defences threatening Nato planes. But that would be an appropriate response to Russia’s deliberate escalation. Though less likely, Russia might decide to use actual chemical weapons (such as mustard gas or sarin nerve agents), biological agents, or even a nuclear weapon. After all, Russian military doctrine foresees the possibility of escalating a conflict to convince the other side to back down. And Putin’s determination to win makes using any of these weapons a conceivable option. Though a Nato response would risk even further escalation, not responding would risk the future credibility of Nato and its capacity to deter. An effective response need not include responding in kind nor require a full-on war against Russia. But if Russia uses chemical or biological agents, let alone a nuclear weapon, Nato should come to the direct defence of Ukraine – deploying its air, ground and naval forces to ensure Russia’s defeat and the full restoration of Ukraine’s independence and sovereignty. No one wants to start world war three. But Putin should know that if he escalates, Nato’s calculation of risks and rewards will change. Letting Moscow know that Nato would come to Ukraine’s direct defence under these circumstances would not only be legal and legitimate, but necessary to demonstrate to Russia and others that escalation does not pay. Ivo Daalder is the president of the Chicago Council on Global Affairs. He served as US ambassador to Nato between 2009 and 2013