President Donald Trump has never seemed particularly concerned with building public support for the war in Iran. Although the president and his administration have offered a succession of rationales for why the United States is now six months into a conflict that has cost at least $35 billion (likely much, much more) and has no end in sight, they have devoted little sustained effort to making the case with the U.S. public. What began as a mission to eliminate Iran’s nuclear capabilities has evolved into a campaign to keep the Strait of Hormuz open, even though the waterway—which is crucial for global trade—was open before the war began.

Historically, most presidents take seriously the task of building public support for military action and all the related challenges that come with mobilizing. Even when wars prove extraordinarily controversial or problematic—and sometimes when they are justified on the basis of false information—presidents have understood the need to articulate a clear theory of the case so that the public knows why U.S. blood and money are being spent.

President Donald Trump has never seemed particularly concerned with building public support for the war in Iran. Although the president and his administration have offered a succession of rationales for why the United States is now six months into a conflict that has cost at least $35 billion (likely much, much more) and has no end in sight, they have devoted little sustained effort to making the case with the U.S. public. What began as a mission to eliminate Iran’s nuclear capabilities has evolved into a campaign to keep the Strait of Hormuz open, even though the waterway—which is crucial for global trade—was open before the war began.

Historically, most presidents take seriously the task of building public support for military action and all the related challenges that come with mobilizing. Even when wars prove extraordinarily controversial or problematic—and sometimes when they are justified on the basis of false information—presidents have understood the need to articulate a clear theory of the case so that the public knows why U.S. blood and money are being spent.

When presidents fail to do this, as is the case with Trump, and conditions on the ground deteriorate, as is the case with Iran, they often find themselves in political trouble. If the public never feels invested in the operation, the war effort will quickly lose support. Under the best circumstances, wars are extraordinarily difficult and costly, not only in terms of money and human lives but also because they divert resources and attention from other major challenges facing the nation. With Iran, the efforts overseas have actually worsened one of Americans’ biggest concerns: affordability.

President Franklin Roosevelt, who served from 1933 until his death in 1945, understood that success in the court of public opinion was an essential component of waging war effectively. Even in a conflict........

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