In November, terrorists struck at the heart of Paris. Such attacks are meant to terrify, but they will only strengthen our resolve.

We are facing the biggest security challenges in a generation. They are complex, interrelated and come from many directions. The promise of the Arab Spring has been replaced by violence and extremism. Russia has illegally annexed Crimea, it continues to destabilize eastern Ukraine and has now entered the war in Syria.

NATO is adapting to this changed world.

Last year, we took the decisions needed to keep our nations safe by increasing our readiness and investing more in our defense. 2015 has been a year of action. We are completing the implementation of NATO's Readiness Action Plan, the biggest increase in our collective defense since the Cold War.

We have increased our military presence in Central and Eastern Europe. We are setting up eight new small headquarters to support planning, training and reinforcements. We have more than doubled the size of the NATO Response Force to more than 40,000 troops. At its core is the new high-readiness Spearhead Force, ready to deploy within days to wherever needed. This year, NATO allies are holding some 300 exercises, including the largest in over a decade, recently held in southern Europe.

To respond effectively to hybrid threats, NATO must be able to monitor, assess, react and respond in real time. So we are improving our intelligence and early warning, speeding up our decision-making and enhancing our cyber defenses.

But many of the challenges we face cannot be tackled by military means alone. So we are working with the European Union and other partners to project stability in our neighborhood, but we need to work even more closely together.

NATO is actively engaged in the fight against terrorism and extremism. All NATO allies contribute to the global coalition to counter the Islamic State group. We are ?sharing analysis, information and intelligence. We are also helping our partners to better defend themselves and to contribute to regional stability, through defense capacity-building in countries such as Iraq and Jordan, and our mission in Afghanistan.

In our eastern neighborhood, we are supporting countries like Ukraine, Georgia and Moldova to become stronger.

In December, we invited Montenegro to begin accession talks to become the 29th member of the alliance. We expect Montenegro to continue reforms, and to further build public understanding of NATO membership and support for it. Montenegro's NATO membership will contribute to stability in the western Balkans and send a clear signal that the door of our alliance remains open.

We are also reviewing our relationship with Russia. There is no contradiction between having a strong NATO and engaging with Russia. On the contrary, the best basis for a constructive dialogue is a strong defense.

So now is the time to invest in our defense. We have seen some progress toward each NATO nation spending 2 percent of gross domestic product on defense. Estonia and Poland have now reached this mark, the UK is increasing its defense spending in real terms, and other allies have laid out a road map to do the same.

In the last couple of years, our world has become more dangerous. But in the same period, NATO has adapted to make sure we are still able to keep our countries and our people safe. This is what we are determined to do as we prepare for our summit next July in Warsaw.

Stoltenberg is NATO's secretary-general.

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