The next president of the United States must lead in the restoration of NATO’s power and purpose to defend Europe and North America from the most serious security challenges since the end of the Cold War.

Transatlantic leaders must confront a jarring reality: the peace, security, and democratic stability of Europe can no longer be taken for granted. The transatlantic community faces four fundamental strategic challenges—a revanchist Russia, eroding stability in the greater Middle East, a weakened European Union, and uncertain American and European leadership—that threaten the entire community, and by extension global security.

Russia is no longer...

The United States has a direct stake in Europe’s security, prosperity, and freedom. Since President Truman committed the United States to NATO in 1949, the United States has understood that a free and democratic Europe, allied to the United States, is a vital American interest, fundamental to the preservation of the contemporary global security order we led in creating. After twice having to come to Europe’s aid during the twentieth century through the bloodiest wars in history, every American president for six decades has understood the advantages of investing in and preserving Europe’s peace.

What a great bet that has...

Russia is the primary cause of this new threat to NATO. Russia’s illegal annexation of Crimea in 2014, its cynical war that divided Ukraine, and its support for the murderous Assad regime in Syria undermine the liberal international order and endanger security in both Europe and the Middle East.

At the Munich Security Conference in February 2016, Russian Prime Minister Dmitry Medvedev said “Speaking bluntly, we are rapidly rolling into a period of a new cold war.” Following the conference, many analysts and observers were quick to downplay or dismiss his comments. But allies should not to fooled. President Putin...

The United States must also take steps to reinforce its own commitment to leadership in NATO. Over the last decade, US leaders have moved NATO from the center to the margins of US national security policy. Our alliance is only as strong as the allies’ commitment to it—and this begins with the United States, its natural leader. NATO has strategic direction when the United States leads the Alliance to align and mobilize our allies. NATO is effective when the United States works through the Alliance rather than circumvents or even undercuts it. NATO is not an instrument to which...