In July 1939, Franklin D. Roosevelt met with senators from both political parties at the White House in a final effort to persuade them to amend the Neutrality Act preventing America from aiding other countries. After drinks were poured, Roosevelt and his secretary of state, Cordell Hull, argued that the world was approaching a catastrophic war. The 74-year-old Republican senator William Borah, who had led the fight against Woodrow Wilson and American entry into the League of Nations in 1919, shook his head in disgust. “There is not going to be any war in Europe this year,” he said. “All this hysteria is manufactured and artificial.” Two months later Hitler invaded Poland, and England and France declared war on Germany.

Now that it has become the good war fought by the greatest generation, the ferocity of the disputes over entering World War II has largely been forgotten. But the story of America’s anti-­interventionist lobby is not only historically fascinating, it also echoes in debates today over whether America should engage abroad or hold back. The historian Arthur Schlesinger Jr. — whose memoir, Philip Roth said, inspired his novel “The Plot Against America,” about an alternative reality where the isolationists, led by Charles Lindbergh, defeat Roose­velt for the presidency — recalled the dispute as the “most savage political debate in my lifetime,” eclipsing those over McCarthyism and Vietnam in its intensity.

The debate was largely rooted in disappointment over the outcome of World War I, when Wilson’s promised crusade for democracy ended with the punitive Treaty of Versailles. Leading liberal historians like Harry Elmer Barnes and Charles Beard, both of whom had noisily championed Wilson’s decision to intervene, now denounced it. The Harvard Crimson declared in an editorial, “We refuse to fight another balance-of-power war.” And after Joseph Stalin signed a nonaggression pact with Nazi Germany in 1939, American Communists obediently heeded Moscow and denounced Roose­velt as a warmonger.

At the same time, senators like Gerald P. Nye, who had headed an investigation into the munitions manufacturers of World War I (“merchants of death”), attacked the idea of bailing out “British plutocrats.” What’s more, appeasers like Henry Ford, Joseph P. Kennedy and Lindbergh called for cooperation with the poor misunderstood Nazis, while The Wall Street Journal pleaded for “realism” in a June 1940 editorial, arguing that Hitler had “already determined the broad lines of our national life for at least another generation.” Just as American Communists hailed the progress represented by the Soviet Union, so appeasers on the right saw Hitler’s fascism as the inevitable wave of the future, even as they denounced Roosevelt’s New Deal totalitarianism.

“Those Angry Days,” by Lynne Olson, a former White House correspondent for The Baltimore Sun and the author of several books on England and World War II, and “1940,” by Susan Dunn, a professor of humanities at Williams College, powerfully recreate this tenebrous era. Olson captures in spellbinding detail the key figures in the battle between the Roosevelt administration and the isolationist movement. She maintains that the president was too timorous in challenging Congress, but the fervor and depth of isolationist sentiment suggest a more sympathetic verdict. Far from shirking the conflict, Roosevelt played his cards well, seizing upon events to nudge the country toward war and patiently waiting, as he told Winston Churchill, for the big crisis that would settle the debate. Dunn superbly depicts the 1940 election between Roosevelt, who was seeking an unprecedented third term, and his internationalist Republican opponent, Wendell Willkie.

It was Willkie, more than any other Republican politician, who ended up challenging the party’s embrace of isolationism, but this did not really occur until after the election, when he traveled to Britain with Roosevelt’s approval and was promptly denounced as a “Republican Quisling” by Col. Robert McCormick, the rabidly isolationist publisher of The Chicago Tribune. To the consternation of mossback Republicans, Willkie had captured the nomination by riding a groundswell of enthusiasm for an outsider. As a candidate, however, he began to hedge on interventionism. So, Dunn shows, did Roosevelt.

Olson argues persuasively that Roose­velt drew a lesson from his failed Supreme Court packing scheme in 1937 (the opposition to it was spearheaded by Senator Burton K. Wheeler, the Montana Democrat) and his inability to defeat Republicans in the 1938 Congressional elections: he could never get ahead of public opinion. Olson also reports that numerous high-ranking officers in the Army, Navy and Army Air Corps sought to sabotage Roosevelt and that “just before Pearl Harbor, Hap Arnold, the Air Corps chief of staff, was implicated in the leak of one of the administration’s most closely guarded military secrets — a contingency plan for all-out war against Germany.”

In the Senate it was none other than Wheeler who denounced Roosevelt’s modest attempts to keep Britain afloat as it single-handedly battled Germany. When Roose­velt backed a bill for conscription in 1940, Wheeler was apoplectic: “If this bill passes, it will slit the throat of the last great democracy still living — it will accord to Hitler his greatest and cheapest victory.” Members of right-wing groups like the Congress of American Mothers traveled to Washington dressed in black to scream and spit at recalcitrant legislators and hang an effigy of Senator Claude Pepper wearing a sash inscribed with the words claude “benedict arnold” pepper.

Olson shows that the campaign against the isolationists was successfully waged by several prominent citizens’ groups, including members of New York’s Century Association, who called themselves “Centurions.” These establishment worthies, led by the lawyer Grenville Clark, enjoyed close contacts in the Roosevelt administration. Clark persuaded the Republican statesman and interventionist Henry Stimson to join Roose­velt’s cabinet in June 1940 as secretary of war. In addition, Frank Knox, a Republican and the publisher of The Chicago Daily News, joined as secretary of the Navy. Both were drummed out of the Republican Party at its national convention.

The most nettlesome antagonist Roose­velt faced was Lindbergh. He presented himself as a cool and dispassionate realist, assuring his American audiences that England was doomed and that there was no choice but to cozy up to the Third Reich. But he tipped his hand at an America First rally in September 1941 in Des Moines, when he announced that the real enemy was internal and Jewish — “their greatest danger to this country lies in their large ownership and influence in our motion pictures, our press, our radio and our government.”