In 1962, Avis was in search of a new advertising campaign. Since its inception, the car rental company had trailed behind the market leader, Hertz. So the ad agency Doyle Dane Bernbach decided to embrace Avis' second-place status as a sneaky way to tout the brand's customer service. "When you're only No. 2, you try harder" went the new tagline. "Or else."

The "We Try Harder" ads were an instant hit. Within a year, Avis went from losing $3.2 million to earning $1.2 million — the first time it had been profitable in more than a decade. From 1963 to 1966, as Hertz ignored the Avis campaign, the market-share percentage gap between the two brands shrunk from 61-29 to 49-36. Terrified Hertz executives projected that by 1968 Avis might need a new ad campaign — because it would no longer be No. 2.

Acknowledging any sort of brand weakness used to be anathema to Madison Avenue. Perhaps it's no coincidence that a woman devised the Avis slogan. DDB copywriter Paula Green — a real-life Peggy Olson — came up with the "When you're only No. 2" construction. It was revolutionary because, as Green said in later interviews, "It went against the notion that you had to brag."

Even after the '60s zeitgeist faded, advertisers continued to recognize the genius of the DDB approach. You see it in Pepsi's long-running tagline, "The choice of a new generation," which positioned Coke as the choice of establishment fogeys. You saw it in Apple's "1984" Super Bowl spot, in which a lone, colorful rebel dares to resist a monochromatic horde of IBM users, and in Virgin Airlines' cheeky campaigns, in which a fun and sexy upstart thumbs its nose at staid and respectable airline brands.

Green's insight into the power of humility remains influential. Consider the Domino's campaign from a few years back, in which the pizza maker allowed that its crust used to taste like "cardboard." Or think of ads for the Bing search engine that acknowledged the brand's perceived inferiority to Google.

How does the Avis and Hertz story end? The "We Try Harder" assault went unanswered for years as Hertz tried to float above the fray. But in 1966 — hemorrhaging market share, its back against the wall — Hertz began to fight back. "For years, Avis has been telling you Hertz is No. 1," read the copy on the first response ad. "Now we're going to tell you why." The market-share gap stabilized by 1969, settling in at a steady 48-35.

In ensuing decades, the two firms continued to battle. The past year has seen an acquisitions arms race: Hertz bought Dollar Thrifty, and Avis snapped up Zipcar. But the tone has mellowed. The feud isn't nearly as personal or intense.

Avis never did catch up to Hertz. Instead, Enterprise passed them both. Because Hertz and Avis had focused so fiercely on the airport market, they became vulnerable when first 9/11 and then the recession slammed the air travel industry. The car rental business came full circle: Suddenly, Enterprise's slew of downtown locations — used by people whose cars are in the shop or who don't own cars — offered a less cyclical, more profitable model. Enterprise made smart agreements with insurance companies, funneling in business from folks in need of loaner cars after auto accidents.

Hertz is No. 2 these days, and Avis lags in third place. It was a terrific 20th-century rivalry. But in the end, trying harder for five decades straight must have gotten a little exhausting: Last year, Avis finally dropped the "We Try Harder" tagline from its ads.