By Phil Stewart and Jarrett Renshaw

PHILADELPHIA (Reuters) -Three years before retiring from the U.S. Army in 2017, Donald Trump-backed Pennsylvania gubernatorial candidate Doug Mastriano posed in Confederate uniform for a faculty photo at the Army War College, according to a copy of the photo obtained by Reuters.

The previously unreported photo, released by the War College to Reuters after a request under the Freedom of Information Act, showed Mastriano in a 2013-14 portrait for the Department of Military Strategy, Plans, and Operations, where he worked.

Faculty at the time had been given the option of dressing as a historical figure, people familiar with the photo said. At least 15 of the 21 faculty in the photo opted to appear in regular attire. Although one man wears a trench coat and sunglasses and another carries an aviator's helmet, Mastriano is the only one wearing a Confederate uniform.

Mastriano did not immediately respond to requests for comment made by email and phone. A Reuters reporter attended a Mastriano event on Wednesday to seek comment, but the candidate did not make himself available for questions.

Displays of Confederate symbols can be seen as insensitive to those who view them as painful reminders of racial oppression and the Civil War that saw 11 rebelling Confederate states fight to keep Black people enslaved.

The U.S. military issued a de facto ban on displaying the Confederate flag and has sought to remove segregationist symbols from bases and academic institutions following the murder in May 2020 of George Floyd, a Black man whose killing by a white police officer in Minneapolis triggered protests worldwide.

After Reuters made its formal request for the photo, it was removed from the War College wall where it had hung alongside other annual portraits of faculty groups.

The Army War College (AWC), a premier military higher education institution in Carlisle, Pennsylvania, told Reuters a team had reviewed all art, text, and other images displayed at Carlisle Barracks in 2020, but missed the faculty photo.

"The faculty photo did not get the team's attention; the photo has since been removed because it does not meet AWC values," the college said in a statement.

Asked about the War College photo, a spokesperson at U.S. Army headquarters said: "The Army supports commanders who remove symbols or images that do not comport with Army values."

Confederate symbols and dress have been embraced by white supremacists in the United States, and monuments and flags honoring the Confederacy have been removed from many public areas in recent years.

Pennsylvania plays an outsized role in U.S. politics as a so-called swing state in presidential elections, and Republican Mastriano, who has embraced Trump's stolen election lies, is trailing his Democratic opponent in the governor's race ahead of the November ballot.

It is unclear how the photo might be viewed in Pennsylvania, which played a critical role in the Civil War. More than 33,000 soldiers from Pennsylvania died fighting for the Union, and the state was the site of the Battle of Gettysburg, the conflict's bloodiest battle, which ended with a Union victory and inspired President Abraham Lincoln's Gettysburg Address.

Mastriano's district, Pennsylvania's 33rd, includes Gettysburg.

Many Americans continue to participate in Civil War battle re-enactments, wearing uniforms from both sides in an effort to preserve U.S. history.

Jenna Ellis, a senior adviser to Mastriano's campaign and former Trump lawyer, said the media was having a "melt down" because Mastriano once posed as a civil war historical figure for a photo.

"And? He has a Ph.D in HISTORY," Ellis wrote on Twitter. "The left wants to erase history. @dougmastriano wants us to learn from it. I invite @Reuters to go on a Gettysburg tour with Doug. You'll learn a lot!"

(Reporting by Phil Stewart and Jarrett Renshaw; editing by Heather Timmons and Daniel Wallis)