Mao-chan

Mao-chan is a 2002 original anime by Studio Xebec, written by Ken Akamatsu (post-Love Hina, pre-Mahou Sensei Negima) and directed by Yoshiaki Iwasaki. A follow-up manga was illustrated by RAN.

What do you do when Japan is under attack by Ridiculously Cute Aliens? Fight back with even cuter kids! Mao Onigawara, Misora Tsukishima, and Sylvia Maruyama, three third-graders, are tasked with protecting Japan's national landmarks from the aliens... and keeping their grandfathers, the heads of the Ground, Air, and Naval Defense Forces, respectively, from fighting with one another about which one is superior, a problem that often gets in the way of their jobs.

Tropes used in Mao-chan include:

Anti-Villain: Yuriko, who really doesn't want to have to fight Mao-chan and the others.
Beary Cute: In episode 18, the cute alien of the week is a black bear with a white muzzle... that promptly gets lost in a snowstorm.
Big Ol' Eyebrows: Sylvia
Blush Sticker: Misora
Catgirl: Half-alien spies Chinami Noki and Yuriko Ozora, whose ears and tail come out when they get flustered. This happens quite a bit to Yuriko.
Child Soldiers: Well, technically. Done in the most adorable way possible, of course.
Clothing Damage: Happens to Yuriko when she and Mao are ingested by a snake alien. This is Ken Akamatsu we're talking about, after all.
Cuteness Proximity: Everyone gets like this whenever the cute aliens show up, which is part of what makes them a threat — it's difficult to defend yourself or the world when you're being distracted by cuteness.
Defeat Means Friendship: But not necessarily in that order.
Disproportionate Retribution: The new prime minister was bullied by the heads of the defense forces back in grade school...so he undermines the Defense Corps with a male trio of elementary school children who manage to get to the scene of every alien sighting first. This is because the Prime Minister's lackeys are sabotaging the girls...and also because the aliens aren't real.
The Ditz: Sylvia is very spacey.
Dojikko: Mao
Easy Amnesia: Chief Onigawara gets caught in an explosion and suffers amnesia, thinking he's 20 again and mistaking Kagome for his late wife. Hilarity Ensues.
Expy
- Kagome-sensei looks an awful lot like Naru, especially in Chapter 8, when she's not wearing her glasses and has her hair down... and is wearing the exact same outfit Naru was wearing in volume 7 of the manga. No, seriously, it's exactly the same.
- There's also a one-shot appearance by a very obvious Keitarô expy. Like, so thinly disguised you'd think they were Writing Around Trademarks. Unlike the real Keitarô, "Keinosuke" failed the fourth attempt at getting into Tokyo U, though.
- One last one: among the many ridiculously cute animal-shaped aliens is what appears to be an expy of Tama-chan.
The Faceless: Mao's parents and grandmother.
Friendly Enemy: Yuriko really likes Mao-chan and her friends. Unfortunately, as Chinami keeps reminding her, they're supposed to be the villains. Note the "supposed to be" - by the end of the story, it's obvious Yuriko was Good All Along.
Giant Enemy Crab: Chapter 21.
Girlish Pigtails: Mao
Giving Someone the Pointer Finger: Chief Onigawara
Headbutt Thermometer: Mao uses this to check Yuriko's temperature in the manga. She is unaware of Yuriko's heart going all doki-doki.
Heel Face Turn: After Chinami and Yuriko get captured by their bosses as punishment for their failures, Chinami decides to try to help Yuriko to escape and live a normal life, as she is really a good person. Yuriko insists that Chinami come along, too, and both of them join the forces of Japan in fighting off the aliens.
Honesty Is the Best Policy: The commanders knew all along that the girls had gone to Hokkaido, and Mao-chan's honesty keeps her from getting punished.
Hot Springs Episode: At a run-down place that looks an awful lot like Hinata-sou. Complete with Narusegawa Naru, although they aren't specifically named.
Idiot Hair: Sylvia
In the Name of the Moon: Each of the girls has one. Mao: "I will defend Japan in high spirits today!" Misora: "I will protect the skies of Japan!" Sylvia: "Japan's seas are my seas!"
Ineffectual Sympathetic Villain: Chinami. Yuriko's not even villainous enough to be this.
Interservice Rivalry
Joke Weapon: Used by the Chief of Operations: a giant squeaky hammer.
Kansai Regional Accent: Sylvia
Killer Rabbit: all of the aliens are extremely cute. The "Chief of Operations" turns out to be like this as well, and less than a foot tall. And then there was the "legendary alien that can't be controlled or tamed", Omega...which turns out to be Mao's lost cat Mi-kun.
Large Ham: Chief Onigawara
Lethal Chef: Mao's cookies, made with cement mix instead of flour.
Lolicon: Hmm... actually, Yuriko's feelings for Mao seem to border on Foe Yay -- not that Mao realizes that Yuriko is her enemy. Mao/Yuriko actually seems like an even more viable ship than Mao/Misora (see below), actually, because it looks like Mao has feelings for Yuriko as well. Wait, are we sure that CLAMP wasn't somehow involved with this?
Magical Girl: Complete with a transformation that puts Mao in a majorette uniform and does nothing else... until she finally gets a mobility boost.
Mega Neko: The alien from Episode 1.
Meganekko: Chinami.
Monster Protection Racket: The new Prime Minister, to promote his "Three Aces" team. Turns into a game of Zany Scheme Chicken when the Defense Force leaders launch their own fake alien at the same time to try to get the Defense Corps girls back in the public's good graces and independently, Yuriko launches a real alien for the girls to fight... and all three aliens look like pandas.
Monumental Theft: The aliens work their way up from stealing various easily-portable items to stealing an entire Shinkansen train, to stealing the Tokyo Big Sight.
Omake: Two: One that takes place 10 years later, and then a "summer vacation special" after the higher-ups had been introduced.
Paper-Thin Disguise: Chapter 20, when the girls sneak off to Hokkaido.
Psychopathic Manchild: Emperor Galactica.
Reverse Mole: A Defense Corps spy has been on board the aliens' mothership all along. Later revealed to be Mao's father.
Rose-Haired Girl: Mao
Screw the Rules, I Have Money: You will not believe some of the things that Sylvia can afford.
Shout-Out: Episode 20 is one long Lampshaded shout out to Love Hina.
Student Council President: Yuriko.
Subordinate Excuse: Kagome has a crush on Chief Onigawara. If these feelings were ever returned, it would be a May-December Romance, as she's 27 and he's in his late 60s.
Take Our Word for It: That alien in episode 14 of the anime was too cute!
Theme Naming: The Three Aces are Haru, Natsu, and Tou -- Japanese for spring, summer, and winter.
The Thing That Goes Doink: One of the many cultural items targeted by the aliens.
Transformation Sequence
Transformation Trinket: The smiling clover pins. Unusual in that the Defense Forces obtained them from the enemy.
Verbal Tic: Misora ends her lines with arimasu, which is translated as "don'tcha know" and "if you please", while Sylvia's casual manner of speech is translated with the liberal use of "yo" and calling everyone "dude/dudette".
What Measure Is a Non-Cute?: This trope is exactly why the aliens are so tough to take down, and why the military has to use a special force of 8-year-olds to accomplish the task.
Winged Humanoid: Misora when she transforms. Justified as her plane doesn't have a cockpit, so she needs something to help her fly.
X Meets Y: Let's see, this series is about a group of politicians and grizzled military types who recruit three children to battle against strange creatures that have come from outer space to threaten the safety of Japan. And they do so using technology stolen from the enemy, and with much destruction of the scenery. Yeah, it's sort of like if Neon Genesis Evangelion were put under the Moeification ray.
You Gotta Have Blue Hair: Misora