Susan Dunn is the Massachusetts Professor of Humanities at Williams College and the author of "A Blueprint for War: FDR and the Hundred Days That Mobilized America" (Yale University Press, 2018). The opinions expressed in this commentary are those of the author. View more opinion articles on CNN.

(CNN) By attacking our closest allies and their leaders and displaying obsequious deference to the virtual dictator Vladimir Putin, Donald Trump has repudiated American global leadership. He has replaced it with a hyper-nationalism and unilateralism that are less of a foreign policy and more the bullying tactics of a would-be strongman.

Susan Dunn

Trump's "America First" isolationism is fast weakening and isolating the United States, undermining the stability of long-standing alliances, and allowing dictatorships to thrive unchallenged around the world.

At the same time, it represents a dangerous and head-spinning reversal from decades of American conduct. Today, more than ever, we need Republican leaders who share the far-sighted and generous internationalist vision of the post-war future held in the 1940s by three exceptional Republicans: Henry Luce, the publisher of Time, Life, and Fortune; Wendell Willkie, the successful Indiana businessman who ran against and lost to Franklin D. Roosevelt in the 1940 election; and Henry Stimson, FDR's secretary of war.

Luce published his famous article, "The American Century," in February 1941, seven months after all the European democracies -- except Great Britain -- had surrendered to Hitler's ruthless armies. Luce traced this catastrophe to the end of World War I when the Senate rejected Woodrow Wilson's League of Nations and the United States began its retreat from world affairs.

In the 1920s and '30s, Americans embraced "the moral and practical bankruptcy" of isolationism, Luce wrote, and "failed to play their part as a world power." That unwillingness to confront Hitler's aggression in the 1930s, he underscored, brought "disastrous consequences" to "all mankind."

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