t’s always a pleasure to be in Berlin, and I’m glad to be with you today to address the security challenges facing our Euro-Atlantic community – challenges that have been driven home anew by the horrific terrorist attacks in Paris over the weekend.

For much of the twentieth century, Berlin symbolised the Cold War between the liberal democracies of the West and the Communist dictatorships of the East. The Wall, which for so long divided this great city, was the physical embodiment of that confrontation – the Iron Curtain rendered in concrete.

During that time, NATO existed to perform one function: to keep our people and our territory safe from Soviet attack, and thereby provide the security that underpinned our freedom and our prosperity. We did this by being strong militarily and united politically, preventing any possible threat from the Soviet Bloc. This was demonstrated through NATO’s doctrine of deterrence, the subject I would like to focus on today.

Deterrence is a relatively simple idea. It’s about convincing your opponent that the costs to him of attacking you will outweigh any potential gain – that the costs will be so high, in fact, as to make any attempt not only not worthwhile, but a terrible mistake.

During the Cold War, deterrence worked. The Soviet Union knew that any attempt to attack NATO would be met by a swift and overwhelming conventional response and, potentially, a nuclear one. The cost would be failure at best and potential annihilation at worst. NATO was able to convince Soviet leaders of this due to a number of vital factors that we had in place.

First, there was the clear political will on the part of all the Allies to act together as one. There was no doubt that if one Ally were attacked, then all the Allies would respond. Soviet leaders couldn’t just pick off one or two smaller nations without fear of consequences; they would always have to deal with every Allied nation, and that included the United States.

Second, it was obvious that we could back up our words with deeds. We had the troops, we had the equipment, we could demonstrate their quality through exercises, and ultimately, we had our tactical and strategic nuclear forces to make up for any perceived asymmetry in conventional capabilities. We were not limited to one action or another; we could choose, from many, the most appropriate and most effective response – a response that could increase in severity if that were needed.

This flexibility, this ambiguity of our response, produced uncertainty in the minds of Soviet generals and political leaders, making any calculations significantly more difficult.

And third, we communicated a clear and consistent message: that we were ready, willing and able to act to defend our Alliance. There was no ambiguity about that. This message was delivered through diplomatic channels, in public announcements, and in our military exercises, demonstrations and force posture.

Each of these three pillars of our deterrence was essential. Without the political will to act in unity, all our equipment and declarations would have been pointless. Without a strong and capable military, our solidarity and clear communications would have been of no value. And without making the unacceptable costs clear in the minds of the Soviet leadership, our deterrence would have failed.

It was only by having all three parts in place, at all times, that our deterrence succeeded. Moreover, it not only prevented the Cold War from descending into World War III and contained Soviet expansionism; deterrence also created a level of stability that, even with the ideological conflict of the Cold War, enabled us to engage in dialogue and cooperation in certain areas, including arms control and confidence-building measures.

In short, deterrence paved the way for détente, and introduced predictability into a still-competitive relationship.

Then, in the late 1980s, history moved into fast forward, with the arrival of glasnost andperestroika, and with the possibility of moving beyond containment and beyond traditional notions of deterrence.

If a divided Berlin symbolised the Cold War, a newly united Berlin symbolised the optimism and hope of the post-Cold War period – a hope that was crystallised when thousands of people came out onto the streets to cross newly opened borders and smash that terrible wall 26 years ago this month.

When Communism collapsed across the Eastern Bloc, the enemy we had been so vehemently deterring suddenly no longer posed a threat. Our efforts switched from deterring Russia and its involuntary allies in Eastern Europe, to welcoming them as friends and partners. The two Germanys became one and many members of the old Warsaw Pact sought and found membership in NATO.

The world had changed. This did not mean that deterrence was no longer important. But the specific threat had subsided – from a real and present danger, to a more abstract notion of a potential threat from an unknown aggressor.

This more benign security environment in Europe enabled NATO to gradually shift the focus of our forces away from deterrence and collective defence towards greater flexibility and being able to deploy our forces quickly around the world. NATO exercises and NATO operations dealt increasingly with conflicts beyond our borders in places such as Bosnia, Kosovo and Afghanistan. The nature of our equipment, our training, and our expertise shifted too.

This shift in focus was reflected in our updated Strategic Concept, agreed at our Lisbon Summit in 2010. It spoke of the continued importance of deterrence and collective defence. But it also emphasized, in the age of globalisation, the need to protect our interests further afield – be it by deploying our forces beyond our borders to protect our security at home, as we did in Afghanistan; or by supporting our partners in their efforts to maintain stability through cooperative security.

The 2010 Strategic Concept codified what the Alliance had been busy doing for the previous two decades in response to a changed security environment.

And now our security environment has changed again. In just a couple of years, our neighbourhood has been plunged into turmoil and violence by many varied causes. The promise of the Arab Spring has turned to dust, leaving a trail of failed or failing states in its wake from Libya to Syria. Terrorist groups like ISIL have been quick to fill the vacuum, spreading bloody violence across North Africa and the Middle East – and even onto our streets.

And as ISIL’s reign of terror continues and as Syria collapses, millions have been forced to flee for their lives, prompting a humanitarian catastrophe and the greatest refugee crisis Europe has experienced since World War II.

And now Russia has also entered the conflict. Russia’s military build-up in Syria, its air strikes and its cruise missile strikes, are not mainly aimed at ISIL, as the US-led coalition’s forces are. They focus instead on supporting the continuation of the Assad regime. Russia still has the ability and the opportunity to make a constructive contribution to ending the war in Syria and destroying ISIL. But as things stand now, its actions are only pro-longing the war and the suffering of the millions of people caught up in it.

In Ukraine, it is now almost two years since Russia deployed its “little green men”, denying their activities until Crimea was illegally annexed and brought fully under Russian control. Since then, it has continued to support so-called “separatists” in eastern Ukraine with soldiers masquerading as “volunteers” and “vacationers,” with weapons – including heavy weapons – and with command and control.

While we have seen some progress in implementing the Minsk agreements, the risk remains of a resumption of violence by Russia and its proxies at any moment.

Through its aggressive actions in Ukraine, Russia has ripped up the international rule book which we had all worked so hard for so many decades to write: rules that ensured the sovereignty of nations and the sanctity of borders; that ensured that disagreements would be solved through diplomacy and negotiation and not on the battlefield; that stated that every nation had the right to chart its own course and to choose its own destiny.

Since the end of the Cold War, NATO worked hard to include Russia, and not to isolate her. Our aim was a strategic partnership where we could work together. As a result, borders were opened, trade flourished and, over time, trust increased.

We signed the NATO-Russia Founding Act and created the NATO-Russia Council. We cooperated on counter-terrorism and counter-piracy, and on helping Afghanistan. And we offered to work together on areas such as missile defence.

All of this benefitted us, and it benefitted Russia. But today, the choices made by Moscow have taken our relations with Russia to their lowest point in decades. We are not back to the Cold War, but we are far from a strategic partnership.

In recent years, Russia’s military activity at the Alliance’s borders has increased significantly. We have seen a military build-up in Kaliningrad, in Crimea and now in Syria. Russia has the ability to move massive numbers of forces quickly along its borders, and their anti-aircraft and anti-ship missiles cover huge areas of NATO territory. This so-called “anti-access and area denial” (A2AD) capability is designed to restrict our freedom of movement and navigation. It is something we are paying close attention to in our planning.

Our security environment may have evolved in the last couple of years, but so has NATO. Since our Summit in Wales last year, Allies have been busy implementing the biggest increase in NATO’s collective defence since the Cold War: the Readiness Action Plan (or RAP). This has strengthened our ability to respond with great speed and tremendous power to any kind of attack – threatened or actual – from any point on the compass.

In the run-up to our next Summit, in Warsaw next July, we will continue adapting and bolstering our deterrence posture.

Our security environment today is complex and fast moving. It is more dangerous and less predictable than it has been for decades. We face threats from state and from non-state actors; from the south and from the east; from conventional military forces and from unconventional terrorist, cyber or hybrid attacks.

So we must modernise our deterrence with better intelligence and early warning. We have to speed up how we take decisions, and how we implement them. We must significantly improve our cyber defences. And we must strengthen coordination with other organisations that have a role in countering cyber and hybrid threats – the EU in particular.

Militarily, modernising deterrence means building on the RAP with greater mobility, with cutting-edge capabilities, and better integration of our land, sea and air forces. We need to be sure we have the capacity to reinforce our Eastern Allies now and in the future in the face of Russia’s growing A2AD capability, especially in the Baltic and Black Sea regions. And we need a realistic assessment of our requirements for the pre-positioning of equipment, enablers and forward stationing of combat units on a rotational basis so that we can counter even the most devious hybrid attacks.

And beyond strengthening our forces and our procedures, we must ensure that our political unity remains rock solid; that our militaries are strong and capable; and that all potential adversaries understand, loud and clear, that every square metre of this Alliance is defended.

All in all, we need to make clear that, if attacked, we can and will defend every Ally. But it is better to deter those who would attack us from doing so in the first place. Prevention is always better than cure.

Prevention may also be the key to countering the threats from the South, where our adversaries – non-state actors like ISIL – may not be susceptible to traditional concepts of deterrence. We must prevent ISIL from capturing more territory and roll back the gains it has already made. And we must help build the defence and security capacity of our neighbours in the Middle East and North Africa so that they can prevent their nations from becoming ISIL’s next victims.

Coming back to Europe, let me say that I know deterrence is not always a popular word. For some it contains echoes of the Cold War that we would rather not hear in the modern world – as if to deter is in some way an act of aggression or belligerence. I disagree entirely.

Being strong enough to prevent others from attacking you is not an act of aggression. NATO Allies have never had strong military forces because we wanted to fight a war, we have them because we want to prevent a war. Deterrence is not a concept for a bygone age. It is as relevant today as it has always been.

And by ensuring effective deterrence against a revisionist Russia, we will have a more solid basis on which to engage Moscow – to bring it back into compliance with international law and, in time, begin to rebuild ‎the trust and partnership that Russia has destroyed.

Ladies and gentlemen,

Today’s challenges are very different from the ones we faced when this city was divided. And they are very different to those in the decades since it has been reunited.

But NATO continues to evolve as NATO always has. What remains unchanged are our central goals: to protect our territory and our people; to preserve our values of freedom, democracy, human rights and the rule of law; to project stability in our neighbourhood; and to preserve our vision of a Europe whole, free and at peace, within a safe and strong Euro-Atlantic community.

With a strong and effective deterrence, NATO has maintained our security since the Second World War. With a modern, 21st-century deterrence posture, it will maintain that security for many more decades to come.