Members of the New York State Assembly had a good laugh on April 18, 1882, when their youngest member showed up dressed like Prince’s great-grandfather.

“He looked like the parody of a Victorian dandy: hair curled and parted in the middle, muttonchop sideburns, a thin mustache, and a monocle in one eye . . . His formal morning coat opened to reveal skin-tight bell-bottom trousers. In one hand he carried a gold-tipped cane, in the other a silk top hat.”

This remarkably attired upstart was 23-year-old Theodore Roosevelt, the future US president whose very name was later taken to indicate a toughness of spirit, but whose colleagues in the assembly referred to him as “Weakling,” “Jane-Dandy” and “Punkin-Lily.”

One newspaper even asked if he was “one of those men ‘given to sucking the knob of an ivory cane.’”

“The True Flag” looks at the debate, at the turn of the 20th century, around whether the US should become a nation of empire. Roosevelt became the leading voice for imperialism, but as author Stephen Kinzer depicts him, he was also an ambitious, scheming warmonger whose reputation as a fierce soldier was more myth than reality.

When Roosevelt lost his wife and mother on the same day in February 1884, he “reacted by plunging into frenetic action,” including buying a cattle ranch and hunting.

“It was therapy by playacting, the rich boy living out his fantasy of rugged life,” writes Kinzer.

One who thought little of Roosevelt’s supposed outdoorsiness was Mark Twain, a later opponent in the expansionist debate.

“He has no sympathy with any brand of nature study other than his own,” Twain wrote. “In a word, Mr. Roosevelt is not a naturalist, but a game killer.”

In 1896, Roosevelt was named assistant secretary of the Navy. When word arrived two years later of a possible war in Cuba against Spain, he pulled a fast one to help make it happen. One afternoon, his boss, John Long, left early. So Roosevelt called his pal and fellow imperialist, Sen. Henry Cabot Lodge.

The duo began “dashing off telegrams to naval units at home and abroad,” ordering warships to prepare for battle. When Long discovered what had happened, he chose to let the situation play out. Roosevelt won a clear victory through his shady handiwork.

‘To complete the archetypal rite — to prove his full manliness, especially to himself — Roosevelt needed to kill an enemy soldier.’

As war began to seem inevitable, Roosevelt quit his job to fight in Cuba. But while Roosevelt thought he’d make a great soldier, “few agreed,” as various generals and governors refused his request for a military appointment. He finally found a territorial governor in Arizona willing to make him a lieutenant colonel and to commission him to put together a fighting unit.

The assembled team, known as the Rough Riders, was a military Bad News Bears — a motley collection of cowboys, students, intellectuals and others who didn’t fit into a traditional unit.

The Rough Riders moniker followed Roosevelt into history as proof of his rugged manliness. But Kinzer writes that the group barely saw action and that Roosevelt’s own “heroism” was anything but. “The Rough Riders fought twice,” Kinzer writes, noting that the first battle — in which Roosevelt was second in command — lasted 90 minutes. He also might have been too excitable for the job.

Kinzer writes that the colonel in charge “calmly gave orders.” Meanwhile, Roosevelt “jumped up and down, literally,” reported the New York Journal. “I mean with emotions evidently divided between joy and a tendency to run.”

The second was the Battle of San Juan Hill, which saw “his 400 Rough Riders dislodge a hundred-man Spanish position.” Roosevelt called it “the great day of my life.” But despite his victory, something was missing.

“To complete the archetypal rite — to prove his full manliness, especially to himself — Roosevelt needed to kill an enemy soldier.”

After the battle, “two Spanish soldiers jumped from a trench” and fired on Roosevelt and his aide. They missed, and Roosevelt “drew [his gun] as the two Spaniards turned to run.” He fired several times, missing one but killing the other.

Later, Roosevelt “took pains to emphasize that he had not shot the fleeing soldier in the back but rather ‘in the left breast as he turned.’ ”

Whatever the reality, the Rough Rider image followed Roosevelt home and solidified him as a national hero. He went on to become governor of New York and the US vice president before becoming the youngest president in our history, at age 42, after President William McKinley was killed in 1901.

Even as he reached this pinnacle, detractors remained unimpressed.

“We have never had a president before who was destitute of self-respect and of respect for his high office,” Twain wrote. “We have had no president before who was not a gentleman; we have had no president before who was intended for a butcher, a dive-keeper or a bully.”