Let's concede that the three presidents he lists, Roosevelt, Truman, and Kennedy, were pretty "tough." FDR saw the threat of fascism and led the nation in combating the threat, Truman set up the framework that helped the U.S. maneuver through the Cold War, and Kennedy navigated us through some dire crises. However, these presidents were not guided only by a tough stance that made them "unafraid to make moral judgments." They made choices that evinced an understanding of the strengths and limits of U.S. power and, in their best moments, a realization of the complexities and ambiguities of the world.
Even as Franklin Roosevelt commanded the U.S. through its' most epic conflict, he also recognized the limits of American power and did things that belie a simplistic idea of always standing up to "evil." Roosevelt did nothing to prevent Spain's fall to fascism and until fairly late hoped the U.S. would not have to be directly involved in the war, but rather that it could support England and other nations. Then, of course, there's Yalta, where the U.S. proved unable to stop Soviet domination of Eastern Europe. While many right-wingers of yesterday and today still see Yalta as an unforgivable sign of weakness (appeasement anyone?) as Robert Dallek writes in his magisterial chronicle of Roosevelt and foreign policy, Yalta reflected a recognition of reality: "The price for Soviet defeat of Nazi armies, diplomat and historian George F. Kennan has said, was their control of Eastern Europe."
As for Truman, it is certainly true that he and Acheson took a tough line with the Soviets, up to and including the war in Korea. Yet, they also understood the world's complexities. Acheson may have warned, as Lieberman quotes, that "no people in history have ever survived, who thought they could protect their freedom by making themselves inoffensive to their enemies." However, a look at, for example, his handling of the Iran crises after World War II shows that such a belief did not cancel out an appreciation for diplomacy and nuance. While the British believed that military intervention was needed to prevent President Mossadeq from destabilizing the region, Acheson managed to help stave off this course, realizing that although intervention could potentially "solve" the problem of a government challenging U.S. economic and strategic interests, such a move carried a greater risk of "driv[ing] Iran to a Communist coup d'etat" and that Mossadeq was someone to be negotiated with.
As for John Kennedy, while his inaugural address was certainly "tough" it's also not the only speech he gave. In June 1963, after having been through disasters and great successes in his conduct of foreign policy, Kennedy gave a speech at American University showing a new tone in his thinking about foreign policy. While the speech contained many a "hard-headed" line about combating the Soviets, it also demonstrated a real nuance regarding our seemingly implacable foes (who, after all, had promised to "bury us.") In describing how Soviet leaders perceived the U.S. as a dangerous imperialist power bent on destroying the Soviet Union, Kennedy warned (in a statement reminiscent of Jimmy Carter's concerns over an "inordinate fear of communism," a line Lieberman makes fun of in his column) that:
[I]t is sad to read these Soviet statements, to realize the extent of the gulf between us. But it is also a warning, a warning to the American people not to fall into the same trap as the Soviets, not to see only a distorted and desperate view of the other side, not to see conflict as inevitable, accommodation as impossible, and communication as nothing more than an exchange of threats.Lieberman makes a number of other points of minimal historical insight, such as confusing the views of the 1960s New Left with the Democratic Party. The main point however is the one made by Kennedy above: foreign policy is not morally simplistic undertaking and leaders who history has judged kindly are usually the ones who realize that.