Nissan has been vowing for two decades to get serious about the American full-size pickup market with a product that challenged Ford, Chevrolet and Ram.

But the Japanese automaker is now taking a step back — a reminder that challenging mighty Detroit's pickups isn't so easy.

Nissan told its U.S. dealers last week that it will discontinue sales of the Cummins diesel engine Titan XD and also jettison certain other Titan configurations, including its single-cab models.

The moves are minor plays within the segment for Nissan. But they are symbolic. Nissan trumpeted the arrival of the Cummins V-8 diesel Titan in late 2015, making it the emblem of the brand's second attempt to shoe-horn its way into the competitive full-size pickup market.

Nissan executives and planners said Nissan's first-generation Titan in 2003 was a low-volume affair because the nameplate lacked key variations for serious truck buyers. Without workhorse street cred, such as a V-8 diesel, they said at the time, Nissan would not be a contender for real pickup cross-shopping.

But Nissan's efforts have gone unrewarded.

The reality is even more stinging in light of the current U.S. market: Pickup sales have been booming, and the Detroit 3 have been maneuvering this year to increase their truck output.

For the first six months of this year, the Titan mustered just a 1.5 percent share of the full-size pickup segment, with sales of 18,026. That was a decline of 23 percent from a year ago.

Nissan's walk-back underscores the powerful hold that Detroit's brands have on the pickup segment. Ford, for instance, sold 448,398 full-size pickups through June.

"Full-size pickup buyers buy American brands," said Sam Fiorani, vice president at AutoForecast Solutions. "This segment is as close to American as apple pie and the bald eagle. This is the Detroit 3's bread-and-butter market, and no Johnny-come-lately is going to tap into it."