Pepsi has suffered many setbacks in recent years, but perhaps few were as symbolic as being jilted by a long-time partner it used to own, KFC.

The fried chicken chain said last week that it would begin replacing PepsiCo’s namesake soft drinks with Coca-Cola products at its 3,700 U.S. restaurants in January. The two companies’ partnership stretches back 40 years, since the time PepsiCo bought the fast food chain. (PepsiCo spun KFC off in 1997.) KFC represents a tiny sliver of Pepsi sales, but the defection reflects the ongoing deterioration of the drink’s brand power .

How Coke pulled ahead

For decades, the Coke vs. Pepsi cola wars were a part of mainstream culture, with memorable ads showcasing blind “Pepsi Challenge” taste tests of the two drinks. Pepsi’s TV commercials with pop star Michael Jackson and model Cindy Crawford in the 1980s and ’90s became part of the larger zeitgeist. Back then, Coke’s U.S.…