After the attacks of September 11, 2001, our NATO allies did more than signal their support for us. They invoked Article 5, the provision of the treaty supporting the principle of collective defense, for the first—and, so far, only—time in the history of the alliance. They joined us on the battlefields of Afghanistan to bring those responsible for that heinous attack against the United States and the American people to justice. They partnered with us for the past two decades on counterterrorism and security operations throughout the world. More than 1,000 men and women from our allied partners gave their lives in this conflict for our freedoms. The United States Congress and the American people will never forget their sacrifice.

This is why more than three in five Americans view NATO favorably. This is why we are leading an effort in the Senate to pass legislation that would require the consent of two-thirds of the United States Senate for any present or future president of the United States to be able to exit NATO.

In an alliance as broad as NATO—with its 29 members, 21 partners for peace, and an additional 15 nations engaged in formal programs—there will always be differences. For instance, some of Turkey’s recent actions have raised serious questions regarding Ankara’s intentions and commitments to certain fundamental democratic principles, such as human rights and press freedoms. Yet we must seek to resolve these differences within the alliance, not outside of it.

For long-term sustainability, it will be important for all NATO members to increase their spending on defense, in order to meet and exceed the minimum threshold of 2 percent of GDP. We strongly believe that as allies, we should have frank but respectful conversations about this important issue.

NATO should also devote more resources to the challenge of hybrid warfare, including protecting our cyberspace and countering information operations that seek to damage our democratic systems. NATO should also become more global and use both its Partnership for Peace program and its formal dialogues around the world to grow beyond its North Atlantic focus.

As Winston Churchill famously said: “There is only one thing worse than fighting with allies, and that is fighting without them.”

This is why we will always stand with NATO.

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Tim Kaine has served as United States Senator from Virginia since 2013 and was the Democratic party’s nominee for Vice President in the 2016 election.