The Earth is under dire threat, from beings which come from the sky, look biological, and attack Japanese cities for no adequately explained (at first) reason. Only three children can pilot the semi-sentient mechanical instruments of destruction created by the secret Japanese military organisation for this precise purpose.
Unfortunately, the kid around whom the series centres is rather lacking in self-confidence, and gets things wrong then starts apologising endlessly.
She's also eight years old, and called Mao-chan. And she's sworn to defend Japan from every "Kawaii Alien-san".
Definitely not. Although I'm sure such crossover fics do exist, EarthDefenderMaoChan is not such a thing. Instead, it features a demented Army General grandad who's like Eva's Gendo except he wears a cape and shouts, a schoolteacher looking very like LoveHina's Narusegawa Naru except she's got a hopeless crush on the aforementioned general, two more eight-year-old schoolgirls with magical-girl transformation sequences who defend Japan's sky and sea respectively, with the help of mysterious clover badges (which do get explained later), a student union president with cat-ears which keep popping out at inopportune moments, and lots more madness...
It does have quite a bit in common with Centurions, now you mention it. But not in the way that the three 8-year-old girls are (understandably) quite incompetent as military heroines, much to the distress of their ambitious grandfathers! *grin* --AC
You may think after one or two episodes that this is nothing but sickeningly-cute kiddy anime. You'd be wrong, although it probably helps if you can find the first episode or two funny.
It is a requirement that before the Universe can end, Nagi must be forced to watch Episode 3 of EarthDefenderMaoChan. Possibly any other episode would do, but the cuteness would probably make Nagi dismember those trying to force him to watch it. Episode 3 contains certain references which ought to make Nagi laugh hard enough that his attempts to slaughter are temporarily interrupted.
It's important to remember that everyone in this series is utterly incompetent. The girls, obviously; their teacher and the generals; the aliens, the press, the older schoolgirls, the secret agents, everyone. AlexChurchill has been caught out more than once by presuming some slight measure of competence on the part of... someone, anyone. There are also bucketloads of references to NeonGenesisEvangelion and LoveHina.
But not everything is ladled on with a trowel. As well as more subtle references to series other than the two mentioned, they do fun things with characters' identities, pulling new meaning later on out of aspects which have been there since episode 1. There are a number of hinted references which are explained further through towards the end, as well as some others which stay completely surreal and unexplained :-)