Once again, a flag has become a symbol of division.

Less than a month after a massacre in Charleston by a white supremacist spurred nationwide debate over the Confederate flag, another mass shooting has fueled spurious flag rumors and spawned criticism of President Obama.

On Tuesday, the president ordered all American flags on federal grounds to be lowered to half-staff for the remainder of the week to honor the five military service members killed on Sunday in Chattanooga, Tenn.

But for some critics, Obama’s announcement came too late.

“Oh one more thing, lower the FLAG!!!!!!!! Sir,” former Navy SEAL and “Lone Survivor” author Marcus Luttrell wrote on Monday on Twitter.

On a Facebook page titled “Half-Mast Challenge; Teach Obama Respect,” scores of people — including conservative commentator Glenn Beck — had already submitted photos of their own flags at half-staff.

Even Donald Trump preempted the president, ordering flags on his own U.S. properties lowered on Tuesday.

Citing false rumors that Obama had lowered the flag when pop singer Whitney Houston died — in fact, it was New Jersey Gov. Chris Christie who did her the honor — retired pitcher Curt Schilling slammed the president on social media.

Where the Charleston shooting had stirred soul-searching over the place of the Confederate flag in America, the Chattanooga massacre risks turning the Stars and Stripes into a political punching bag.

Partially obscured by the controversy is the reason why the flag is lowered in the first place.

The flag is to be flown at half-staff “when the whole nation is in mourning,” according to the Department of Veterans Affairs.

“By order of the President, the flag shall be flown at half-staff upon the death of principal figures of the United States Government and the Governor of a State, territory, or possession, as a mark of respect to their memory,” according to the United States Code. But he has the discretion to lower it as well “whenever he deems it to be appropriate or desirable.”

Recent presidents have “deemed it appropriate” for, among others, Pope John Paul II, Rosa Parks, Neil Armstrong, former House Speaker Thomas Foley and the victims of the Virginia Tech and Newtown shootings, the Indian Ocean tsunami and the Boston Marathon bombings. Half-staff flags were flown for foreigners Winston Churchill, Anwar Sadat, Yitzhak Rabin and Nelson Mandela, among others.

Among the earliest examples of a half-mast flag in the United States was in 1799, when the Navy Department ordered all Navy vessels to “wear their colours half mast high” on the occasion of the death of George Washington.

But why half-staff? Look to history.

“According to one line of scholarly thinking, by lowering the Union Jack, the sailors were making room for the invisible flag of Death,” reports Mental Floss’s Ethan Trex. “This explanation jibes with the British tradition of flying a ‘half-staff’ flag exactly one flag’s width lower than its normal position to underscore that Death’s flag is flapping above it.”

The practice dates back to at least 1612 and an ill-fated mission.

At the beginning of the 17th century, King Christian IV of Denmark sent three successive missions to Greenland to locate long-lost Norse settlements and to claim the massive island for his country. During the first expedition in 1605, Scottish explorer John Cunningham (sailing for Denmark) captured four Inuits and took them prisoner.

The Inuits did not forget.

And so it was that seven years later, when Englishman James Hall led another, privately funded expedition to the same area in Greenland to look for silver, the Inuits sought their revenge.

“While Hall was in the boat, he was struck by a spear and killed,” according to “Exploring Polar Frontiers: A Historical Encyclopedia.”

First-person accounts of the incident include some of the first known references to lowering a flag to half-staff or, in this case, half-mast. Quartermaster John Gatonbe, who had remained behind on the main ship, Heart’s Ease, saw Hall’s smaller, expeditionary boat — called a pinnace — coming and knew something had gone wrong.

“This day, at night, came our vice-admiral, with our great pinnace at her stern, her flag hanging down, and her ancient [colors] hanging down over her poop, which was a sign of death,” Gatonbe wrote in an account republished in “Danish Arctic Expeditions, 1605 to 1620.”

“We, being most of us asleep but the watch, were soon awake, for our pinnace came aboard of us and told us of the death of our master and general, James Hall, and how with a dart he was slain of one of the savages, and died the 23d of July,” Gatonbe continued.

Flags fly at half-staff in Bayonne, N.J., as smoke rises from lower Manhattan following the collapse of both towers of the World Trade Center in New York, Wednesday, Sept. 12, 2001.  Two hijacked commercial aircraft crashed into the center's towers Tuesday. (AP Photo/Charles Krupa)

Flags fly at half-staff in Bayonne, N.J., as smoke rises from lower Manhattan following the collapse of both towers of the World Trade Center in New York, Sept. 12, 2001. Two hijacked commercial aircraft crashed into the center’s towers on Sept. 11. (Charles Krupa/AP)

Like now, there was back then a political power struggle, with a fight breaking out over who should take command of the ship. Thankfully, however, the bickering eventually gave way to agreement and the Heart’s Ease, under the command of a young captain named William Huntriss, lowered its own Union Jack to half-mast and headed home.

“The Heart’s Ease entered the Thames on the 19th of September, and Huntriss caused the flags to be hoisted half mast, in token of the death of his beloved commander, James Hall,” Clements R. Markham wrote in the notes to “The Voyages of William Baffin: 1612 – 1622.”

The lowered flag was then, as it should be now, a symbol of shared loss: something that unifies viewers in respect for the dearly departed.

Not a symbol of division.

“We sail’d up the river of Thames, the wind being easterly,” Gatonbe continued. “And so, before London, in St. Katharine’s pool, we anchor’d, having our flag and ancient [colors] hanging down, in token and sign of the death of Mr. Hall, our general.”