A sense of dread swirled over Baltimore as Maj. George Armistead took command of Fort McHenry in 1813.

British ships had been scudding through the Chesapeake Bay. Baltimore, the young nation’s third-largest city and a “nest of pirates,” was in their crosshairs.

Armistead wanted a flag to fly over the fort, a big one. A flag to show the British what they’d be dealing with. And he knew just the person to make it.

Mary Pickersgill, one of the city’s few female business owners, ran a flag-making shop from her Jonestown neighborhood home a few blocks from the harbor, stitching ensigns for sailboats and “colors” for the many militias headquartered around Baltimore during the War of 1812.

Pickersgill, whose 250th birthday will be celebrated Thursday, was about to stitch herself into history.

The flags that she and her helpers constructed would wave over the fort during the Battle of Baltimore, inspiring Francis Scott Key to write the poem that would become the nation’s anthem.

“I consider Mary Pickersgill the heroine of the War of 1812,” said Sally Johnston, president of the board of the Star-Spangled Banner Flag House & Museum.

The museum is throwing Pickersgill a semiquincentennial birthday party on Thursday, with guest speakers, traditional dances, cake and tours of the home where Pickersgill lived for five decades and began work on her most famous flags.

It’s a chance to celebrate Pickersgill, who also helped to found a retirement home bearing her name that still operates in Towson. It’s also an opportunity to contemplate Pickersgill’s legacy and the women and girls who helped her stitch the famous flag.

“She created a national symbol that is etched in our collective mindset,” said Robert Stewart, the assistant superintendent of the Fort McHenry National Monument and Historic Shrine and speaker at Thursday’s event.

Pickersgill’s big birthday is also a good excuse to visit the Flag House, which the city purchased in 1927 and is run as a museum by a private foundation.

An adjoining visitors’ center includes relics found in and around the home, including porcelain doll eyes, cowry shells used by enslaved people as currency and — not surprisingly — thimbles, pins and scissors.

“This is a museum dedicated to women’s labor history,” said executive director Chris Sniezek. “This place is dedicated to Mary and the other women who made the flag, to give them agency over their life stories.”

Pickersgill’s story begins with that of her mother, Rebecca Young, who opened a flag-making shop in Philadelphia after her husband’s death. Young Mary worked alongside her.

When Pickersgill’s own husband died, she followed in her mother’s footsteps. Pickersgill, her young daughter and her mother moved to Baltimore in 1806 to be closer to family. They started making flags in their brick three-story home at the intersection of Albemarle Street and what was then called Queen Street, but we now call Pratt.

Pickersgill and her mother placed newspaper ads inviting “military gentlemen” to purchase “Silk Standards & Cavalry Colors, and other Colors of every description, finished in compleat order.”

Other residents of their home included an enslaved person whose name was not recorded and Grace Wisher, a 13-year-old “free girl of Colour,” who worked as indentured servant for Pickersgill “to learn the art and mystery of Housework and plain sewing,” in exchange for room, board and $12 per year, according to her indenture agreement.

When Armistead expressed his desire for a “flag so large that the British will have no difficulty in seeing it from a distance,” two of his top aides, Commodore Joshua Barney and Gen. John Stricker, who both happened to be related to Pickersgill by marriage, offered to ask her in person. (Smalltimore was even smaller in those days.)

The military leaders presented a daunting list of demands.

The garrison flag needed to be massive, 30 feet by 42 feet. There would be stripes, 15 of them. And stars, also 15. Because the flag was so large, it had to be made of a lightweight wool bunting, which was only produced in England — the country we were at war with.

Armistead wanted the stripes to be two feet wide, even though rolls of bunting were 18 inches wide. He also wanted a smaller, more durable flag to fly in storms. And he wanted them both in six weeks.

Pickersgill got to work. It’s unclear how she obtained vast quantities of British bunting, but her brother was a privateer (a citizen authorized to steal enemy goods), Sniezek said.

Pickersgill and her team — her mother, daughter, nieces and young Grace — sewed at first in Pickersgill’s bedroom, which had the best light in the house, and then moved to a nearby Brown’s Brewery, when the larger flag outgrew the family home, Sniezek said. (The brewery burned down during the Great Baltimore Fire of 1904, but a Fairfield Inn now stands near the spot, he said.)

Armistead hoisted the completed flag over Fort McHenry in late summer 1813. The flag flew as the British invaded and captured Washington the following year, torching the White House and the U.S. Capitol.

In September 1814, the British turned their attention to Baltimore, storming North Point and marching to Hampstead Hill, in what is now Patterson Park, were they were rebuffed by more than 15,000 American troops.

One quarter of those defending Baltimore were African-American, and one-third were immigrants, said Stewart, the Fort McHenry assistant superintendent. “But they all rallied around the flag,” he said.

Stymied on land, British forces returned to sea. Early on Sept. 13, 1814, the British began firing cannonballs and bombs at Fort McHenry, an assault which lasted for 25 hours while an intense storm also raged. Pickersgill’s smaller storm flag flew during the battle, growing ragged from wind, rain and bits of shrapnel, Stewart said.

In the morning, the British gave up, defeated by Baltimore. As the enemy ships slipped away, Armistead ordered Pickersgill’s massive garrison flag to be hoisted again.

These twin sights — the battered storm flag flapping through the battle and its majestic counterpart being raised at dawn— inspired Key, who watched the bombardment from the deck of a British ship behind enemy lines.

Key, an attorney, had been sent to secure the release of an American doctor the British had taken hostage. Since both men had overheard discussions of British strategy, they were barred from leaving the ship, and instead watched the flag raptly, hoping their country would prevail.

“Oh say can you see,” Key wrote. That was a sentiment Stewart believes was echoed throughout Baltimore that day, as residents celebrated that their city did something Washington could not—thwart the British.

There is no record of how Pickersgill reacted to the sight of her flag flying over the fort. “Women’s history is often fraught with inaccuracies because it wasn’t preserved,” Sniezek said.

She purchased the home on Albemarle in 1820, becoming one of the city’s few female property owners, though records still listed her son-in-law as “head of household” until his death.

Pickersgill became the leader of a movement to create a home for impoverished widows, the Impartial Female Humane Society. The group’s first home opened on Franklin Square in 1851, and many moves and iterations later, it is now Pickersgill of Towson, which bills itself as the nation’s oldest retirement community.

Pickersgill’s home, where she remained until her death, also went through many iterations, becoming home to a Russian Jewish family, a secondhand furniture shop, a post office, drug store, and a steamship company, among other enterprises, before the city purchased and preserved it.

Today the museum is run on shoestring budget. Sniezek, the executive director, is the only full-time employee, and he shoveled the recent snow away himself.

Yet visitors to the Flag House are still struck by its former owner’s extraordinary deed.

“The flag that she created was seen by all of Baltimore,” said Stewart. “All these folks put aside all those differences and came together to save their home.

“And they were looking to the flag sewn by Mary Young Pickersgill.”

This story has been updated to correct the date Armistead hoisted the completed flag.