In the Jazz Jackrabbit series, the Turtles race are the arch-enemies of the rabbit race, the race of the series protagonists. Turtles are typically portrayed as dumb (with, Devan Shell and the Rebel Turtle as the only exceptions) and willing to do anything Devan orders them to do.
In the manuals and on the boxes of most games in the series there is a reference to classical writer Aesop’s work The Tortoise and the Hare (with Jazz Jackrabbit 1‘s text “Remember the Tortoise and the Hare? 3000 years later, they’re still at it!” being the most famous), in which a turtle wins a race between him and a hare because the hare is so self-confident that he thinks he’ll be able to win even when he takes a nap during the race. When the hare awakes, the turtle has already won the match.
It is however unlikely that this ancient feud is the cause of the games’ war between turtles and rabbits, as turtles both won the race and were the originators of the war (so revenge wouldn’t be a reason to start the fight). The history of the war is never explicitly mentioned in the series and probably it should be regarded as a plot device that allows the creators to put hundreds of mindless turtles in the levels as obstacles, as Jazz Jackrabbit games never have very developed stories and merely focus on fast-paced gameplay.
In Jazz Jackrabbit 1, the turtles have presumably conquered a large part of the universe near Carrotus, residing on numerous planets and employing massive numbers of soldiers. As turtles are typically portrayed as dumb and obedient, this was probably achieved solely by Devan Shell’s leadership and the enormous number of the turtle forces.
Devan Shell was defeated on the planet Lagunicus by Jazz later on, but wasn’t anyhow punished by the hero back then. Therefore, he manages to create a new big army and build a Time Machine. Apart from inhibiting both the past and present, the turtles do not seem to do anything, as unlike in Jazz Jackrabbit 1 they do not have stockpiles of resources, special air bases or anything else built by themselves. As expected, Jazz gets rid of them again.
In Jazz Jackrabbit 3, Devan has risen to power yet again and kidnapped Jazz Jackrabbit‘s children, locking them up in the royal castle. Unlike in other games, the turtles’ conquest is less overwhelming as several parts of the playing world are still in rabbit’s hands. The full story of the turtles in this game is not known, because the only available version of the game was only in early stages.
In Jazz Jackrabbit Advance, which is not regarded as canon by many fans, turtles are a lot more intelligant than in previous games (attacking in groups, organizing, having guns, wearing protective suits and building advanced war machines), but are not lead by Devan Shell, but by Dark Shell. This raises the question whether Devan’s regime was as efficient as thought.
Eat your lima beans, Johnny.