Robert W. Merry is editor of The National Interest and the author of books on American history and foreign policy.

Talk of war is in the air. And as tensions mount between the United States and Iran, it’s proper to assess the lessons of history as they relate to presidential war decisions. The first is that American presidents face no decision more politically dangerous than the choice to take the country into armed hostilities. It can bring glory to the White House occupant, but it can just as easily bring ignominy. Consider the presidencies of Woodrow Wilson, Harry Truman and Lyndon Johnson. All saw their political standing plummet amid war outcomes far different from what had been anticipated in the flush of excitement at war’s beginning. The same happened to George W. Bush, who managed to get through his 2004 reelection effort despite growing difficulties in Iraq. Four years later, however, the full extent of the Iraq mess was evident, and that contributed to the double whammy that befell Republicans in the 2006 and 2008 elections.

Even Abraham Lincoln, generally considered by historians to be the greatest president, nearly came a cropper due to the carnage of war he had unleashed and which, right up until the fall of 1864, still looked like a hopeless stalemate. In fact, just six weeks before the election Lincoln wrote a note to himself, to be stuffed into a drawer until after the balloting. He wrote: “This morning, as for some days past, it seems exceedingly probable that this Administration will not be re-elected. Then it will be my duty to so cooperate with the President elect, as to save the Union between the election and the inauguration; as he will have secured his election on such ground that he can not possibly save it afterwards.”

Between the writing of that note and the election, three military developments occurred: General William Sherman took Atlanta; General Philip Sheridan gained military dominance over the Shenandoah Valley, a key Confederate supply source; and the last Confederate ramming vessel, the Albemarle, was sunk in the Roanoke River, ending rebel resistance to the Union’s naval blockade of the South. These military victories in the field favorably altered the political landscape at home, and Lincoln’s electoral standing soared. Such are the vagaries of war as applied to American politics.

Another lesson might be called the Polk-Johnson-Bush lesson: ensure that no one can ever make an accusation that the president dissembled to the American people in order to get permission to spill American blood...