SOUTH PORTLAND — On May 29, 1919, starlight from the Hyades cluster 153 light-years distant grazed our sun in full eclipse and was deflected 1.61 arcseconds, as measured by British astronomer Sir Arthur Eddington on an island off Africa, proving Einstein’s general theory of relativity. Newton’s universe of absolutes gave way to Einstein’s, in which space and time were relative and motion curvilinear. With absolutes in physics questioned, so, too, were absolute good and evil, and moral relativism emerged, making relativity the “knife … (that) cut society adrift from its traditional moorings in … Judeo-Christian culture.”

So said historian Paul Johnson in “A Relativistic World,” the spectacular first chapter of his 1983 volume “A History of the Modern World.” Johnson surveyed the moral and geopolitical catastrophe of World War I and the intellectual ferment brewed by Darwin, Marx, Freud and especially Friedrich Nietzsche, whose “God is dead” pronouncement presaged the “collapse of the religious impulse” in the 20th century, leaving a huge moral vacuum. “The history of modern times,” Johnson wrote, “is how that vacuum had been filled,” which in the interwar period would be by “gangster-statesmen” with “the Nietzschean will to power” and “an unappeasable appetite for controlling mankind.”

They were stopped by Winston Churchill. If relativity was the century’s greatest idea, Churchill was its greatest man. Disgraced in World War I by the Gallipoli fiasco, he raised the politically incorrect, lonely voice, scorned by intellectuals and elites, that warned of the Hitler menace. Debacle and disaster followed successive British governments that chose to appease until Germans invading Poland forced them to fight. When France fell, Churchill became prime minister.

This story is magnificently told by historian William Manchester in the 1988 biography “Winston Spencer Churchill: Alone, 1932-1940.” Manchester and Churchill are at their best in the final chapter, “Cataclysm.” Paul Johnson wrote, “there are no inevitables in history,” which Churchill demonstrated by rallying Britain from near-certain defeat with words that still echo: “I have nothing to offer but blood, toil, tears, and sweat”; “Upon this battle depends the survival of Christian civilization”; “Let us … so bear ourselves that if the British Empire lasts for a thousand years, men will still say, ‘This was their finest hour.’ ”

Churchill and the British faced absolute evil with courage and moral clarity in a final burst of national greatness that saved mankind from what the prime minister called “soul-destroying tyranny.” Hitler and the Nazis would not fill Paul Johnson’s vacuum.

Nor would godless communism. The United States now carried Western civilization’s torch in the twilight struggle of the Cold War, culminating in Ronald Reagan’s “evil empire” speech and the fall of the Berlin Wall. America bestrode the world a superpower, a benign and reluctant one according to columnist Charles Krauthammer in “Decline is a Choice,” the final and finest chapter of his 2013 collection “Things That Matter.”

Instead of “continuing ascendancy,” Krauthammer saw Barack Obama choosing decline. Apologizing for American “arrogance” and rejecting American exceptionalism, Obama undermined America’s “moral claim … to world leadership,” Krauthammer concluded. Calling for a vast expansion of the social welfare state, Obama undercut America’s ability to project power abroad. And he sided, according to Krauthammer, with those who see America as “so inherently … sinful that it cannot be trusted with … world power.” Obama won’t be judged this century’s greatest man.

How to stop the decline? Krauthammer quoted Demosthenes: “Don’t do what you are doing now.” Enter Donald Trump, who on his first day restored the Churchill bust to the Oval Office, replacing the one that Obama had removed. And then the 45th president began removing all that the 44th president had done, making good the Make America Great Again pledge. If Trump prevails, soulless, socialist globalism, like fascism and communism, will not fill Paul Johnson’s vacuum.

Greatness or decline? Will America choose to remain the dominant, morally confident tip of the spear and soul of Western civilization, with the sinews of power that only a free-enterprise economy can provide, or go the way of socialist Europe, a confederacy of once-great states, emasculated by the decadence of the collective? Will heavenly light a century hence fall on an America strong, proud and free, or on an America in willful decline?