Massive cultural shifts during and after World War I helped free women from confining roles—and the confining corsets that bound them to the previous age. The evolution of the bra re-shaped the image of what a woman could be, whether she was serving in the war effort, fighting for the right to vote, or dancing in a flapper-style dress at war’s end.

“No one person invented the corset or the bra,” says Valerie Steele, Director of The Museum at the Fashion Institute of Technology. “They were developed in different places and many people took out patents over the years improving or changing their design.” Some of the earliest bras date back to Ancient Rome: “Mosaics from the villa Romano del Casals in Sicily show the strophium, a simple cloth breast binding,” says Judith Dolan, distinguished professor and head of design at the University of California at San Diego.

By 1500, corsets—tight, structured undergarments extending from below the chest to the hips—became the undergarment of choice for women in the middle and upper classes in much of Europe. The constricting corset would reign supreme until the 20th century when women began to breathe easier thanks to the bra.

While a 600-year-old prototype of a bra was recently found in a castle in Austria, credit for inventing the first “modern” bra goes to French designer Herminie Cadolle, who cut a corset into two in 1869 and called it the “corselet gorge.” Cadolle’s creation was seen as a bit scandalous at the time. It would take world events—and a patent—for the bra to really take off.

American socialite Mary “Polly” Phelps Jacob patented the “brassiere” on November 3, 1914, the year World War I broke out in Europe. Filing for the patent under the pseudonym “Caresse Crosby,” she’d come up with the concept while dressing for a ball, when her uncomfortable corset poked through her dress, prompting her and her maid to sew together two handkerchiefs to offer more flexible support.

Her business never quite took off (though she’d go on to shake up the publishing world in Paris, printing the work of authors like Ernest Hemingway, Anais Nin, and James Joyce), and she sold her patent to the Warner Brothers Corset Company for a paltry $1,500. By the time the United States joined World War I in 1917, the influence of European fashions and the changing role of women helped open the floodgates for women to ditch their corsets and embrace the bra.

“In 1900, women are wearing corsets and dresses that cover neck to ankle. By the 1920s, hems are at mid-calf, and more women are wearing bras and soft girdles instead of corsets,” says Steele.

French designers had an outsized impact on global fashion, and early 20th-century French designers like Paul Poiret and Chanel championed a straight silhouette, not the hourglass granted by a corset. In his 1931 autobiography, not-so-humbly titled The King of Fashion, couturier Poiret claimed sole credit for the bra, writing: “it was in the name of Liberty that I proclaimed the fall of the corset and the adoption of the brassiere….Yes, I freed the bust.”

Steele refutes this bold claim, citing the hundreds of years of fashion history behind Poiret… and the cultural shifts occurring all around him.

“The influence of World War I really made the bra take hold. Before it was this titillating, different undergarment. World War I changes the role of women around the world,” says Lora Vogt, curator of education at the National WWI Museum and Memorial. “It was a time of tumult that brought on a big, catastrophic shift. People looked at their lives differently and examined how they could give to the larger community. Women began to work in the war industry, ammunition. You can’t do that in a corset,” says Vogt.

Nine million women joined the American war effort in various capacities. “Women serving overseas were given a stipend to take care of their undergarment needs,” says Vogt. And there were quite a few women looking for undergarments befitting their new roles.

By June 1918, over 3,000 Americans were serving as nurses in over 750 hospitals in France. Nearly 12,000 women joined the Navy as yeoman, or non-commissioned officers, working as mechanics, truck drivers, radio and telephone operators, translators, and munitions workers. Seven thousand women applied to be the so-called “Hello Girls,” switchboard operators working for the US Army Signal Corps, 223 of whom were sent overseas.

On the home front, women also stepped up to work in previously male-dominated railroads, on farms, and in factories. Seeing women in these new roles shifted public opinion about what women were capable of, with even President Woodrow Wilson declaring that women’s suffrage was “vitally essential to the successful prosecution of the great war of humanity in which we are engaged.” These new roles required new, more freeing garments… and undergarments.

Women on the home front were asked to reconsider many aspects of their lives, from what food they put on the table to what they wore under their gowns. In 1917, Chairman of the U.S. War Industries Board Bernard Baruch asked women to stop buying corsets. Most corsets of the day were no longer made with whalebone, but with metal stays—metal that could be better used for the war effort. Women were eager to prove their patriotism, and NPR reports that 28,000 tons of steel were diverted, allegedly enough to build two battleships.

As for the corset, though it was eclipsed by the bra in the decades to come, its promise of a slimmer figure never fully went away. As Steele explains, women have constantly adjusted their appearances to the ideal body type of a given era—whether through undergarments or otherwise.

“Women never really gave up the corset," Steele says. "They internalized it through diet and exercise, tummy tucks and liposuction. Once clothing began to show more skin, you had to actually change your body or have a different attitude toward your body."