On June 6, 2019, we marked the 75th anniversary of the D-Day landings. This was a major turning point in the Second World War, leading to the liberation of Europe. And it was a colossal feat for the tens of thousands of Allied troops, many of whom paid the ultimate price for our freedom. Less than 5 years later, NATO was born. An extraordinary idea, driven by the visionary leadership of our 12 founding nations and by the deep desire of our citizens to live in peace and freedom.

NATO leaders will gather Dec. 3-4 in London, the very first home of NATO’s headquarters, at a critical time for our trans-Atlantic security. This is an opportunity to reflect on everything we have achieved over seven decades. In that time, allies in Europe and North America have built an unprecedented area of peace and prosperity. Our ironclad commitment to protect and defend one another guarantees freedom and democracy for our almost 1 billion citizens, making NATO the most successful alliance in history.

But beyond marking those 70 years, leaders will look to the challenges that still lie ahead. Today we face the greatest security threats in a generation — from a more assertive Russia to instability across the Middle East and North Africa, and from cyber and hybrid attacks to the ever-present terrorist threat. At the same time, the global balance of power is shifting around us. And the rapid development of new technology promises to transform our societies — and security — as radically as the first industrial revolution. So we must be ready to respond to any threat from any direction. And that is exactly what NATO is doing.

Since Russia’s illegal annexation of Crimea in 2014, NATO allies have implemented the largest reinforcement of our collective defense since the Cold War. We have strengthened our presence in the east of our alliance, from the Baltic to the Black Sea regions. And we have increased the size and readiness of our forces. This year we declared two new NATO commands operational: one in Ulm in Germany to improve military mobility in Europe, and one in Norfolk in the United States to ensure trans-Atlantic lines of communication. And we are delivering on our new NATO Readiness Initiative to field 30 air squadrons, 30 combat vessels and 30 land battalions within 30 days.

As well as guaranteeing our security today, NATO is preparing for the challenges of tomorrow on land, at sea, in the air, in cyberspace and in space. NATO recently updated the core standards for civilian telecommunications, including 5G, in order to improve the resilience of our networks. And this year allies adopted NATO’s first-ever overarching space policy.

Finally, all allies are stepping up their investment in our security, not only in cash but also with new capabilities and contributions to NATO missions and operations. European allies and Canada have increased defense spending for five years in a row. By the end of next year they will have added an extra $100 billion to their defense budgets since 2016. More allies are on track to meet their pledge to spend 2 percent of gross domestic product on defense by 2024. But we have to do more because our freedom does not come for free. And in a more unpredictable world, we need to continue to keep our citizens safe.

In London, NATO leaders will continue to strengthen our collective defense, modernize our alliance and invest in our shared security. And while NATO’s 70th anniversary gives us cause for celebration, this is not the time for complacency. We must never take the trans-Atlantic bond for granted. We must never take freedom and democracy for granted. We must defend them every day.

Jens Stoltenberg is NATO’s secretary general.