TAYLOR SWIFT SHOWED UP RECENTLY IN NEW YORK CITY’S West Village in just an oversized rugby jersey and thigh-high boots. The look of casual separates with a glamorous twist is nothing new, of course, but a cheeky new appreciation of American sportswear was the reigning trend on the spring runways of designers like Michael Kors, Tory Burch, and Joseph Altuzarra.

Ah, sportswear, that great American fashion homage to freedom, movement, and informality! The term originated in the 1920s as a way to define the comfortable clothes women wore as sports spectators—at the races in Deauville, on the courts at Newport Casino. European designers took inspiration from sporting uniforms like tweed hunting jackets or sailor’s pants. And although the look was considered radical at the time when corsets were still wardrobe staples, Hollywood legends Greta Garbo and Marlene Dietrich picked up on the casual trend and made pants and menswear their signature.

The popularity of ordinary garments didn’t really take off until World War II when, faced with the shortage of expensive fashion fabrics, American designers experimented with more utilitarian clothes like pants, belted jackets, and apron dresses in functional fabrics such as denim and cotton. The idea was to be practical and washable and to allow women to move easily. Thus, crinolines, padding in the shoulders, and darts began to disappear from the designs of an upstart named Claire McCardell, who was making a name for herself in the Seventh Avenue studio of a manufacturer named Townley Frocks.

With the intention of creating a dress for both cooking dinner and entertaining guests, in 1942 McCardell designed the "popover" wrap dress, which she saw as an answer to a magazine's challenge to create an outfit that worked in the kitchen and at cocktails. The dress came with a matching potholder—talk about cheeky sportswear!—and was so popular it landed McCardell a Neiman Marcus Award in 1948 and on the cover of Time magazine in 1955.

Like McCardell, Bonnie Cashin made a name for herself with clothes that allowed freedom of movement and sported practical details including pockets, leather piping, and removable belts. Such mid-century details and wartime functionality find contemporary expression in Tory Burch's pared-down silhouette for spring in a collection of minimal taffeta windbreakers, feather-light organza miniskirts, and sleek polo shirts.

A layer of effortless glamour was added to the pragmatism and playfulness of American sportswear when the society swans of the 1950s and '60s began sporting the look. C. Z. Guest and Babe Paley paired basic crewneck sweaters with taffeta ballgowns, or simple capri pants with silk button-down shirts. The image of Jackie Kennedy Onassis striding through the streets of Capri in 1962 wearing a simple T-shirt and white jeans inspired Michael Kors' spring collection of casual cashmere V-neck sweaters worn with lace skirts or empire-waist dresses with leotard tops. At Proenza Schouler, this distinctly American simplicity was manifested in black satin apron dresses tied nonchalantly low on the hip.

Up until 1973, American fashion ran a distant second to the more fanciful styles on European runways. But when designers such as Halston, Bill Blass, and Stephen Burrows brought their streamlined sportswear to Versailles' Hall of Mirrors in what's now known as the "Battle of Versailles," they won the race for fashion gold against the fussy details of French couture.

Meanwhile, back at home, as women climbed the proverbial corporate ladder, designers like Anne Klein and Liz Claiborne were responding to their real-world needs with functional career clothes that allowed them to look professional and feminine in the office. Instead of dull suits modeled after menswear, women could now embrace color and softer, more feminine silhouettes with the kind of bright, sophisticated separates that still form the foundation of American style. The active 1980s and the introduction of synthetic stretch allowed women even more sartorial freedom, and soon they were wearing activewear-inspired bodysuits and stirrup leggings with structured jackets to the office. Before long, sneakers, yoga pants, and puffer jackets would follow, becoming global staples of designer collections in both America and Europe.

Today it's hard to miss the dominance of sportswear on streets and boulevards around the world. And on runways, the iconic look of casual ease is influencing fashion more than ever.