“Those victories were not as decisive as we remember: It took another 100 years, after the civil rights acts of the 1960s, before the North truly won the Civil War, while the peace that ended the First World War begat the Second World War, and the peace that ended the Second World War begat the Cold War and its many constituent conflicts,” Mr. Exum said. “What Trump is saying resonates because it’s based more on the myths we tell ourselves than the histories written down in long, dense books.”

Former and current American military officials agreed.

“The wars today don’t deliver battlefield victories along the lines of what we saw in World War II, with the surrender on the deck of the battleship Missouri,” said David W. Barno, a retired Army lieutenant general and former commander of American forces in Afghanistan. “We’re fighting enemies with no navies, no air forces or even conventional armies in some cases. Applying only conventional armed forces to these conflicts is not always going to be adequate.”

Several former Pentagon officials, including a number of retired generals and admirals, cautioned against cutting the State Department and foreign aid budgets to help pay for increases in Pentagon spending. In a letter to top congressional leaders, the retired military officers wrote that “elevating and strengthening diplomacy and development alongside defense are critical to keeping America safe.”

“We know from our service in uniform that many of the crises our nation faces do not have military solutions alone,” the generals and admirals wrote. “The military will lead the fight against terrorism on the battlefield, but it needs strong civilian partners in the battle against the drivers of extremism — lack of opportunity, insecurity, injustice and hopelessness.”

James G. Stavridis, a retired admiral who signed the letter, said on Monday that most senior military leaders believed it was unwise to cut development aid and diplomacy funding.

“So often, the far less expensive ‘soft power’ tools — humanitarian relief, medical diplomacy, foreign aid, strategic communications — are under sister agencies such as state and A.I.D.,” said Mr. Stavridis, a former NATO commander who now serves as the dean of the Fletcher School of Law and Diplomacy at Tufts University, referring to the State Department and the Agency for International Development. “Cutting them harshly would be a mistake.”