We face the greatest challenges to our security in a generation. This is no time to question the value of the partnership between Europe and the United States.

For 67 years this partnership has been the bedrock of peace, freedom and prosperity in Europe. It enabled us successfully to deter the Soviet Union and bring the cold war to an end. And it made possible the integration of Europe and laid the foundation for the unprecedented peace and prosperity we enjoy today. European leaders have always understood that when it comes to security, going it alone is not an option.

At the same time, American leaders have always recognised that they had profound strategic interest in a stable and secure Europe. And throughout the last 67 years America has had no more steadfast and reliable partner.

The only time Nato has invoked its self-defence clause, that an attack on one is an attack on all, was in support of the United States after the 9/11 terrorist attacks. This was more than just a symbol. Nato went on to take charge of the operation in Afghanistan. Hundreds of thousands of European soldiers have served in Afghanistan since. And more than 1,000 have paid the ultimate price in an operation that is a direct response to an attack against the United States. Today of all days, we remember them.

On both sides of the Atlantic leaders have always understood that a stronger, safer and more prosperous Europe means a stronger, safer and more prosperous United States. This partnership between Europe and the United States, embodied in the Nato alliance, remains essential for both.

In the last few years we have seen a dramatic deterioration of our security, with a more assertive Russia and turmoil across north Africa and the Middle East. Nato allies have responded together. We have implemented the biggest reinforcement of our collective defence since the cold war. And the United States has significantly strengthened its commitment to European security, deploying a new armoured brigade to eastern Europe and delivering equipment and supplies to support future reinforcements if needed.

This is deterrence, not aggression. We do not seek to provoke a conflict, but to prevent a conflict. Nato battalions numbering thousands of troops cannot be compared with Russian divisions numbering tens of thousands just across the border. Our response is defensive and proportionate. But it sends a clear and unmistakable message: an attack against one will be met by a response from all.

Nato also continues to play a crucial role in the fight against terrorism. Every Nato ally is part of the US-led coalition against Islamic State, our Awacs surveillance aircraft support coalition air operations, and Nato is training Iraqi officers to better fight Isis. We also work with a range of partners throughout north Africa and the Middle East to help them fight instability and improve their security.

The partnership between Europe and America is founded on deeply shared interests and common values. At the same time, a viable partnership depends on all contributing their fair share. The United States currently accounts for almost 70% of Nato defence spending, and has rightly called for a more equitable sharing of the burden.

At the 2014 Wales summit, every Nato ally pledged to stop cuts and increase defence spending to 2% of GDP within a decade. Since then, European allies have delivered, with the United Kingdom showing significant leadership. This year, 22 Nato allies will increase defence spending, leading to a total of 3% increase in real terms. And I expect that next year we will see the third consecutive year of increased defence spending in Europe.

We are an alliance of 28 democracies. Free-flowing debate is part of our DNA. Naturally, we have our differences. But leaders on both sides of the Atlantic, and across the political spectrum, have always recognised the unique ties that bind us. Our proud history is one of common challenges overcome together.

It is all too easy to take the freedoms, security and prosperity we enjoy for granted. In these uncertain times we need strong American leadership, and we need Europeans to shoulder their fair share of the burden. But above all we need to recognise the value of the partnership between Europe and America. It remains indispensable. So rather than deepening our differences, we need to nurture what unites us, and find the wisdom and foresight to work together for common solutions. Going it alone is not an option, either for Europe or for the United States.

Jens Stoltenberg is Nato secretary general and former prime minister of Norway.