NATO is still the world’s strongest military alliance. But its single greatest danger is the absence of strong, principled American presidential leadership for the first time in its history. Starting with NATO’s founding father, President Harry S. Truman, each of our presidents has considered NATO a vital American interest. President Trump has taken a dramatically different path.

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As former U.S. ambassadors to NATO, we interviewed alliance leaders past and present for a new Harvard Belfer Center report: “NATO at Seventy: An Alliance in Crisis.” Nearly all viewed Trump as NATO’s most urgent and difficult problem.

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Never before has NATO had a U.S. leader who didn’t appear to believe deeply in NATO itself. During his first two years in office, Trump has questioned NATO’s core commitment embedded in Article 5 of the alliance’s founding treaty — that an attack on one of the allies will be considered an attack on all. He has been weak and reactive in defending NATO against its most aggressive adversary, Russian President Vladimir Putin.

Trump has also been a consistent critic of European democratic leaders such as German Chancellor Angela Merkel, while publicly supporting anti-democratic populists such as Hungarian Prime Minister Viktor Orban. Trump is the first president to call the European Union a “foe,” rather than a partner, of the United States.

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Fortunately, the vast majority of Republican and Democratic leaders in Congress disagree with Trump on NATO’s value to the United States. They should vote to approve the bills working their way through committees that would reaffirm the United States’ commitment to Article 5 and to require congressional approval should Trump try to diminish our commitment to NATO — or to pull the United States out altogether. Congress would be acting in unison with the public’s strong support for NATO, according to a 2018 poll by the Chicago Council on Global Affairs.

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Critics who agree with Trump present three main arguments for why he is right to question NATO. First, they say NATO’s core job was finished with the end of the Cold War. That ignores, however, Russia’s campaign to destabilize NATO members Estonia, Latvia, Lithuania and Poland. It also ignores Putin’s attacks on the U.S. and European elections in 2016-2018, designed to weaken our democracies from within. Containing Russian power until Putin’s Soviet-trained generation passes from the scene remains a core NATO aim. And, as our report shows, there are new challenges beyond Russia confronting the alliance.

Second, Trump has claimed the allies are “taking advantage of us.” Low European defense spending is indeed a problem for NATO’s future. Germany, in particular, must do much more. But NATO allies have produced real growth in defense spending for four consecutive years, starting with Putin’s annexation of Crimea — a collective increase of $87 billion. On this issue, Trump would be smart to continue to push but while doing so strive to transform himself from chief critic into the unifying leader NATO desperately needs.

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A third criticism is that NATO no longer contributes significantly to U.S. security in the world. Consider the facts: Canada and the European allies came to our defense on 9/11 and invoked the Article 5 mutual-defense clause of the treaty. They viewed Osama bin Laden’s attack on the United States as an attack on them as well. NATO allies went into Afghanistan with us where they and partner nations have suffered more than 1,000 combat deaths. Most of those countries remain on the ground with our soldiers to this day.

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NATO allies have also fought with us in the successful campaign to defeat the Islamic State caliphate in Syria and Iraq. They conduct counterterror operations with us in Africa. The European allies have assumed full responsibility for peacekeeping in Bosnia and the bulk of the burden in Kosovo.

U.S. air and naval bases in allied countries also bring the United States a continent closer to contain Russia in Eastern Europe and confront terrorist threats in the Middle East and South Asia. This is a decisive advantage for the United States. The reality is that NATO is a net plus for the United States in political, economic and military terms.

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In the decade ahead, the United States will fight two battles with authoritarian powers China and Russia. The first is a battle of ideas that will center on Moscow’s and Beijing’s growing confidence in the superiority of their own systems. We will need the full weight of our democratic allies in NATO to repudiate the authoritarian model in this intensifying global debate just as Presidents John F. Kennedy and Ronald Reagan did in the past.

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NATO allies will also be critical in a battle of technology, as the West competes with a more assertive China in artificial intelligence, quantum computing and biotechnology. The United States has a better chance to maintain its qualitative military edge over China if we enlist the scientific and productive capacity of all our allies in Europe as well as in the Indo-Pacific. NATO remains the great power differential between the United States and Russia and China, which have no real allies of their own.

Trump should reflect on a last reality that all his predecessors understood. The United States would be far stronger inside NATO as it faces these challenges than it would be alone. NATO is not just yesterday’s story but is indispensable if Americans want to reach for the elusive goal we have been chasing since World War II: a secure United States alongside a united, democratic and peaceful Europe as its closest global partner.