Millions of people gathered to make a human chain connecting the Estonian capital of Tallinn with the Latvian capital of Riga and the Lithuanian capital of Vilnius.Ints Kalnins / Reuters

Thirty years ago this week, on August 23, 1989, more than 2 million citizens of the Baltic republics of the U.S.S.R. engineered one of the most dramatic and successful mass protests in Soviet history. Men, women, and children linked hands in a continuous human chain more than 400 miles long that they called the “Baltic Way,” connecting the Estonian capital of Tallinn in the north with the Latvian capital of Riga in the center and the Lithuanian capital of Vilnius in the south.

They were protesting what was then the 50th anniversary of one of modern history’s most brutal and cynical backroom deals—the secret agreement made 80 years ago on August 23, 1939—by Adolf Hitler and Joseph Stalin to divide Eastern Europe between them on the eve of the Second World War. The Molotov-Ribbentrop Pact (named after Soviet Foreign Minister Vyacheslav Molotov and Nazi Foreign Minister Joachim von Ribbentrop) divided Poland, giving Hitler a free path to go to war against it 10 days later and Stalin the green light to invade Estonia, Latvia, and Lithuania in May and June of 1940.

The three young Baltic states were stripped of their national identities and incorporated by terror and force into the U.S.S.R. in the summer of 1940. Stalin’s secret police murdered many of the Baltic government, business, and cultural leaders. Thousands of others were sent to the Soviet Gulag prison system east of the Ural Mountains. Against their will, three independent nations were imprisoned as puppet republics of the Soviet Union for more than half a century until they liberated themselves in September 1991, just before the Soviet empire itself disintegrated.

By 2004, in a remarkable transformation, all three were admitted to NATO. They joined the European Union that same year. The story of how these three small countries on the northern rim of Europe made their way from prisoners in the Soviet Union to members of the two great institutions of the West has lessons for us at a time when President Donald Trump is abandoning the American leadership role in Europe that was so critical in bringing the Cold War to a peaceful and democratic end.

President Franklin D. Roosevelt refused in 1940 to recognize the takeover of the three countries by Stalin. He froze Baltic gold reserves and other financial assets to deny their use by the Soviets. Backed by President Dwight Eisenhower, Congress established in 1959 a Captive Nations Committee to illuminate the imposition of communist rule on formerly free nations, including the three Baltic states. Through the long decades of the Cold War, however, very few American officials would have given good odds that the trio would ever regain their independence from Moscow.

When I joined the National Security Council staff in 1990, I became its liaison to the three Baltic legations in Washington, D.C. It was impossible not to admire Estonian Ambassador Ernst Jaakson and Latvian Ambassador Anatol Dinbergs. Jaakson had arrived in the United States as a young diplomatic representative of Estonia in 1929 and stayed all through the lean and seemingly hopeless years of the Soviet occupation of his country to independence in 1991. Dinbergs came to the U.S. in 1937 and, like Jaakson, never left. They came to work every day for more than five decades to represent governments that had ceased to exist at the start of World War II. Along with Lithuanian Ambassador Stasys Lozoraitis, who represented Lithuania in Washington after 1987, they kept faith with their country and the dream that some far-off day in the future, the Baltic states might be reborn. There is simply nothing like it in modern diplomatic history.

Their improbable dream was realized in September 1991 when President George H. W. Bush agreed in an Oval Office meeting to support the reassertion of the independence that had been stolen so cruelly from them by Hitler and Stalin.

The Balts are the real heroes of this story. They liberated themselves against great odds. They did, however, receive critical support from the U.S., Canada, and Europe in the waning months of the dying Soviet Union.

Bush pushed the Soviet leader Mikhail Gorbachev in the summer and autumn of 1991 to let the Baltic states go free, arguing against the use of force by Moscow. When Secretary of State James Baker later visited the three capitals, he pledged full support for their independence.

President Bill Clinton took up the Baltic cause, persuading Russian President Boris Yeltsin to withdraw thousands of Russian troops still in Estonia and Latvia in August 1994, three years after independence had been won. He went the extra mile to get the troops removed by persuading Congress to appropriate funds to construct housing units in Russia for the returning Red Army officers.

President George W. Bush pushed NATO leaders to admit the three Baltic countries into NATO in 2004. As Bush’s ambassador to NATO at the time, I believed the Baltic countries would be truly free only when they were inside the alliance, protected by its Article 5 mutual-defense guarantee.

President Barack Obama continued the bipartisan effort to help the Balts reestablish their freedom and sovereignty. In the wake of Russian President Vladimir Putin’s invasion and occupation of Crimea in March 2014, Obama went to Tallinn’s Freedom Square to warn Moscow about any attempt to invade or undermine the Baltic states. “The defense of Tallinn and Riga and Vilnius is just as important as the defense of Berlin and Paris and London,” he said in a major speech. Obama then made that commitment even more specific: “So if, in such a moment, you ever ask again who’ll come to help, you’ll know the answer: the NATO alliance, including the armed forces of the United States of America, right here, present, now.”

Before the end of his presidency, Obama and NATO leaders deployed a battalion of NATO troops to each of the Baltic countries and Poland as a visible symbol of that commitment—that the independence of the states Russia had dominated in the past would be secure.

The Cold War ended peacefully in large part because of the constancy and determination of the U.S. and its NATO allies. Each American president had a shared sense of what was at stake and a common strategy to deploy U.S. military and diplomatic strength to defend freedom.

Together, they held the line for five decades to help Europe resist communism, even when the odds seemed slim that it would ever be vanquished. John F. Kennedy and Ronald Reagan’s historic speeches at the Berlin Wall best symbolized that common will and commitment.

When the wall finally fell in 1989 and the Soviet Union itself dissolved two years later, President George H. W. Bush proclaimed that a Europe “whole and free” had been reborn and a “democratic peace” had taken root across the continent. This decades-long U.S.-led campaign is surely one of the great foreign-policy achievements in our history. Every American should take pride in it.

President Trump, however, sees the world through a radically different lens than his predecessors did. He is dismantling, block by block, the foundations of our power that made America great from FDR’s time to Obama’s.

His open ambivalence toward NATO has produced a genuine crisis in America’s most important alliance. For the first time in its history, NATO does not have a strong, principled American leader to guide it. Trump is also the first president to call the European Union a “foe,” treating it more as an economic competitor than as a close global partner.

As antidemocratic populists contest power across Europe, Trump has effectively sided with such leaders in Hungary and Italy against true friends such as Germany’s Angela Merkel and France’s Emmanuel Macron. Just this week, Trump bullied the NATO ally Denmark and canceled a state visit to Copenhagen because its government had the temerity to refuse to sell Greenland to the U.S. The reaction among the usually stolid Danes has been anger and bewilderment that an American president would treat them with such disrespect.

And on the eve of this weekend’s G7 Summit, Trump is calling publicly for Russia to be reinstated in the group, even as it continues to occupy Crimea and parts of eastern Ukraine.

The 20th century was the American century not just because the U.S. wielded enormous military and diplomatic power. The U.S. became the leading nation in the world because all of its presidents, until Trump, believed in helping Europe become a united, democratic continent after centuries of war and division.

The lasting message of the Molotov-Ribbentrop Pact is that evil triumphs when democracies fail to stand up to it. And the message of the Baltic Way protests is that America is at its greatest when it stands up for freedom where it is at risk. The Baltic governments that previous presidents worked so hard to defend must now be worried that, if Russia threatens, Trump will not heed those lessons.

As Americans reflect on the 2020 election and the prospect of Trump gaining another term in office, we must come to one simple conclusion: We can’t afford it. Two and a half years into his presidency, Trump has demonstrated beyond a shadow of doubt that America can’t be great in the world, and our allies can’t truly depend on him, while he remains in the Oval Office.

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