Thirty years after the Cuban Missile Crisis, President Kennedy’s defence secretary, Robert McNamara, learnt something that made his blood run cold. Unknown to Washington, in the event of a US attack on Cuba, Soviet forces there had been primed to strike back with nuclear weapons.

Kennedy opted against an attack, but airstrikes had been a clear choice put to him. The idea that this had been contemplated, without knowledge that it would have started nuclear war, was horrifying. The world had edged closer to catastrophe in 1962 than even figures like McNamara realised at the time.

The decades since the Cold War ended have left us complacent about our security. European defence cuts, including in Britain, have gone well beyond the “peace dividend” that followed the dissolution of the Soviet Union. Our policies and strategies have rested on assumptions of peace, and a naive belief that future wars would be elective and waged against weaker states and terrorists. We are led by a political generation that has never had to contemplate confrontations with other nuclear powers.

Now we find ourselves in one. The West has supplied arms to Ukraine and imposed swingeing sanctions on Moscow. The Ukrainians are mounting a fearsome, heroic resistance. China, where a third of corn imports come from Ukraine, might yet encourage Russia to the negotiating table.

But whatever hopes we may have, and however brave the Ukrainians’ defence of their homeland, the weeks ahead will be long and bloody. With Russian military progress slow, its losses mounting, and an assault on Kyiv and other cities still to come, we are sure to see further war crimes and civilian suffering in the coming weeks.

Some hawks have called for a no-fly zone over Ukraine, or at least its western half. This would require Nato to secure the zone by preventing Russian forces from entering or firing into it: in practice, shooting down Russian planes and potentially putting ground troops into Ukraine. Nato has ruled out a no-fly zone, and President Putin says Russia will treat forces trying to impose one as “participants of the military conflict”.

The West can go on arming Ukraine, including by supplying fighter jets, and go further in hitting Russia with economic sanctions. As the war crimes continue, we should, for example, be prepared to stop all Russian energy imports, whatever the short-term cost to our economies. But we cannot become even limited participants in the war without accepting that this will prompt further escalations in the conflict. We can do everything possible to help the Ukrainians fight, but if we join in the fighting – which a no-fly zone entails – we risk a wider European war potentially culminating in nuclear attacks.

The new cold war in which we find ourselves is for several reasons unlike the last. The hostile states we face include Russia, China, North Korea and Iran, all different regimes with different values, interests, geographies and capabilities. We face another threat, much of it homegrown, from Islamist extremists.

Russia and China are integrated to a considerable degree with Western economies. Scientific knowledge, new technologies, the fragmentation of media, easy international travel and the liberalisation of trade have changed the world beyond recognition since the last cold war. We face hybrid warfare encompassing threats from nuclear and conventional forces, and cyber attacks, state-sponsored organised crime and attacks on our democracy and media.

But there remain some similarities with the last Cold War, and lessons we can learn. It is easy to look back now and glibly conclude that nuclear weapons, and the certainty of mutually assured destruction, kept the world safe. There is truth to it, but the reality is more complicated: the world came close to Armageddon more than once, and the peace was maintained, and the cold war eventually won, thanks to strong, but cautious and responsible, leadership.

Unwavering political resolve, confidence in Western civilisation, economic strength, technological advantage, unsurpassable military capabilities, first-rate diplomacy, an understanding of the values, interests and character of the other side, an appreciation that we could not remake the world in our own image, and calm, clear and predictable policies and decision-making all helped keep the war cold.

That achievement, we must remember, also came at a price: military spending ate up a higher percentage of domestic budgets, and millions languished behind the Iron Curtain.

The decision not to commit Western forces to the conflict in Ukraine was made long before Russia invaded. We are treaty-bound to collective defence and a declaration of war if any one Nato member state is attacked, but Ukraine has never been part of the alliance. We must do what we can to support its resistance, and draw clear lines that Putin will understand he cannot cross without consequence.

Finland and Sweden should be invited to join Nato and the alliance should renew its commitment to collective security and nuclear deterrence. If we judge it unwise to extend the alliance to countries such as Moldova and Georgia, we should make clear that we will, if necessary, arm and train their troops, as we have in Ukraine, and punish aggression via diplomatic and economic measures.

Britain should make a commitment to maintain the most powerful armed forces in Europe. Ukrainian resistance is showing the importance of modern technology and weaponry, as well as the old truth that boots on the ground matter. We need economic strength and resilience, shorter supply chains and energy security.

We need to rid London of dirty money, sanction and expel Putin’s oligarchs and kick China out of our vital national infrastructure. We need to stop the self-indulgence, self-loathing and navel-gazing, and behave as though we believe our way of life is worth defending.

Doing so, as the last Cold War teaches us, will take not just strength, but long-term thinking and informed caution too. This is no game, but a highly dangerous confrontation with enemies who have the power to destroy the world as we know it.