In 1932, during a lunch in Albany with Rexford Tugwell, an adviser, Gov. Franklin Roosevelt paused to take a telephone call from Louisiana Gov. Huey Long. When the call ended, FDR referred to Long as the second-most dangerous man in America. Who, Tugwell asked, is the most dangerous? FDR answered: Douglas MacArthur.

As Army chief of staff, MacArthur had just flamboyantly conducted the violent dispersal of the bedraggled “bonus army” in Washington. Nearly 19 years later, he was to become most dangerous to himself, as another commanding general has now done. But Stanley McChrystal is no MacArthur.

MacArthur had some of the genius and much of the egomania of a former artillery captain, Napoleon. This made MacArthur insubordinate and got him cashiered by a former artillery captain, Harry Truman. Although McChrystal is a fine soldier who rendered especially distinguished service in Iraq, there is no reason to ascribe to him either egomania or insubordination. He did, however, emphatically disqualify himself from further military service, and particularly from service in Afghanistan. There the military’s purely military tasks are secondary to the political and social tasks for which the military is ill-suited, and for which McChrystal is garishly so.

The American undertaking in Afghanistan is a fool’s errand and McChrystal is breathtakingly foolish. Even so, he and it were badly matched. This, even though the errand is of the president’s careful devising and McChrystal was the president’s choice to replace the four-star general who had been commanding there.

It may be said that McChrystal’s defect is only a deficit of political acumen. Only? Again, the mission in Afghanistan is much more political than military. Counterinsurgency, as defined by McChrystal’s successor, Gen. David Petraeus and tepidly embraced by Barack Obama for a year or so, does not just involve nation-building, it is nation-building.

This does not just require political acumen, it requires the wisdom of Aristotle, the leadership skills of George Washington and the analytic sophistication of Tocqueville. But, then, the grinding paradox of nation-building is this: No one with the aptitudes necessary for it would be rash or delusional enough to try it.

The McChrystal debacle comes as America’s longest war is entering a surreal stage: The military is charged with a staggeringly complex task, the completion of which — if completion can even be envisioned — must involve many years. But when given the task, the military was told to begin bringing it to a close in a matter of 18 months.

The not quite seven months that have passed since the president announced his policy have seen sobering military disappointments and daunting evidence of how intractable is the incompetence and how manifold is the corruption of the Kabul government. For as long as we persist in this Sisyphean agony, the president will depend on forthrightness from a military commander whose judgment he trusts. That could not be McChrystal. If he had been retained, he would have henceforth been chastened, abject, wary and reticent. It is unthinkable that he could still have been a valuable participant in future deliberations with the president and his principal national security advisers. The president demanded, and the Americans in harm’s way in Afghanistan deserve, better.

It is difficult, and perhaps unwise, to suppress this thought: McChrystal’s disrespectful flippancies, and the chorus of equally disdainful comments from the unpleasant subordinates he has chosen to have around him, emanate from the toxic conditions that result when the military’s can-do culture collides with a cannot-be-done assignment. In this toxicity, Afghanistan is Vietnam redux.

In July 1945, with the war in the Pacific still to be won and Winston Churchill engaged in the Potsdam conference, the British electorate turned him out of office. When his wife, Clementine, suggested that this might be a blessing in disguise, he replied: If so, it is very well disguised indeed.

The shattering of McChrystal is a messy blessing if the president seizes upon it as a reason for revisiting basic questions about whether Afghanistan matters so much and what is possible there and at what cost. It may be said that with the Afghan mission entering — or soon to enter; it is late and now may become more so — a crucial military phase in Kandahar, the cradle of the Taliban, McChrystal is indispensible. Any who may say that should heed the words of another general, one of the 20th century’s greatest leaders and realists. Charles de Gaulle said: The graveyards are full of indispensable men.

George F. Will writes for The Washington Post Writers Group. E-mail address: georgewill@washpost.com.