Those Were Only Their Scouts

The heroes are attacked, and they manage to defeat their foes without terribly much trouble. They congratulate each other, joking about how easy that was, except for one grim and silent member. When asked why, this trope is the response.

Often used on the first instalment of a work to allow a victory against a foe...And then hammer the heroes (and the audience) with a warning that the Evil Overlord will not be overthrown today. This puts the last fight in perspective, as what was a relatively easy fight takes on nightmarish proportions if you consider that those troops were not even intended to be fighters in the first place, yet they gave the heroes some difficulty.

Interestingly, in Real Life, it's common for scouts to be particularly Badass compared to the main force - after all, if the scouts don't return to report back, the main force will be exactly where they started but without the troops that acted as their scouts. So if killing the scouts was so easy, it wouldn't be illogical to assume their main force is a bunch of Cannon Fodder.

Used in basically every Sorting Algorithm of Evil work (Sailor Moon, Ronin Warriors, etc). If said at the end of a work, a very common Sequel Hook.

See also Normal Fish in a Tiny Pond.

Anime and Manga

 * In Macross, the human military think they will defeat the Zentraedi because they don't realize just how massive the Alien armada is.
 * Similarly, in Macross II, they make the same mistake with the Marduk.
 * At the start of Dragonball Z, when Raditz arrives to Earth, it takes the combined efforts of Piccolo and Goku, some help from Gohan, and even a Heroic Sacrifice from Goku to kill him. During his last moments, Raditz tells of his two stronger companions, who will come to Earth in a year.
 * Digimon Frontier: The team finally faces off with the dark angel Big Bad Cherubimon. He throws off their best attacks easily, and after giving each member of the team the opportunity to throw The Worf Barrage at him so he can laugh it off, he smashes everyone with a single attack. Kouichi, The Atoner, gets his Digimon forms back, redesigned and stronger, and manages to defeat Cherubimon... only for the voice of Cherubimon to taunt him. That was just a much-weaker projection the real Cherubimon whipped up in order to try and talk Kouichi back to The Dark Side. The real thing will be waiting for them when they reach his lair.

Comic Books

 * In one issue of Superman, Superman has been training for weeks and had to team up with the son of one his enemies to fight Imperiex. After a hard battle, Superman wins the battle and thinks Imperiex is through and goes home. Sometime later we find out that "Imperiex" was just a probe and that Imperiex Prime was a gigantic version of the thing Superman fought.

Film
"Valentine: Roger that Burt, and congratulations. Be advised, however, that there are two more, repeat, two more motherhumpers."
 * The attack of the Army of Anubis in The Mummy Returns. The Medji think they've defeated the army, only to the realize that the REAL army is swarming over the dunes like a monstrous black shadow.
 * From Tremors:

"Rasputin: Salt, gathered from the tears of a thousand angels, restraining the essence of Sammael, the Hellhound. The Seed of Destruction. This I can promise, Sammael, for every one of you that falls, two shall arise."
 * Invoked in the first Hellboy movie:

Literature

 * In Sergey Lukyanenko's The Stars Are Cold Toys, the mightiest battle fleet of the entire known space is attacked by an unknown aggressor, who deals them heavy damage before being taken down. It turns out, however, that said "aggressor" was a mere scout of the super-advanced Geometer civilization.
 * The Hitchhiker's Guide to the Galaxy's Frogstar Scout Robots.
 * The Warg Rider attack in The Two Towers.
 * At the end of Mutineer's Moon, the first book in David Weber's "Empire From the Ashes" series, there is an Apocalyptic space-battle on the fringes of Earth's solar system between human beings and a race of alien Omnicidal Maniacs. Hundreds of thousands of humans die, and Terran civilization is nearly wiped out, but eventually the good guys win. And its only then, in the last pages of the book, that it is discovered that the invasion fleet humanity nearly lost to was only a scouting force, and the real invasion fleet (with a thousand times the number of battleships than the fleet that just attacked Earth) is still coming.
 * In Aesop's Fables, a lion his hit by an arrow, and runs away from the man who shot him. A fox, seeing this, makes fun of him, and the Lion retorts, "Yes, I'm retreating. If he's sending a messenger like this, just think how tough the man himself must be!"

Live Action TV

 * This is the Starfleet fear during later seasons of Star Trek TNG: what if instead of one cube, the Borg send millions?
 * Used in one episode of Star Trek: Voyager: the ship manager to defeat a Borg scout without much damage, so Janeway suggests they start an offensive against the Borg. Seven of Nine reminds her that the outcome would be different had they encountered a cube.
 * They try to take on a cube anyway with a method that should keep them invisible to the Borg, and a meticulously worked-out plan that will let them steal the Applied Phlebotinum they need and get away in just a few minutes. ...and the Borg Queen reveals to Seven of Nine not only does she know exactly where they are, she could Curb Stomp them any time she wished and would only keep holding back if Seven of Nine rejoined her.

Music
"Prying my switchblade cold out of my fingers' hold, Pause to take stock, reflect, and rue. Look on the damage done here by a single one; What do you think a full pack will do?"
 * In Least Of My Kind by Echo's Children a werewolf invokes this on himself (see the title), just to sour the victory for his killers.

Tabletop Games

 * In Warhammer 40,000, the absolutely massive Tyranid fleets that have ravaged several sections of the Imperium are said to only be the scouting fleets of the main force.
 * The same applies to the Necrons.
 * Tau experienced this with Imperium of Man. PDF and anti-piracy navy detachments? No big deal. Imperium got sufficiently irritated to send a whole crusade, including Space Marine forces? And it turns out even then they barely react due to having bigger problems even in this region? Ah...

Video Games
"Louis: We made it! I can't believe we made it! Bill: Son, we just crossed the street. Let's not throw a party until we're out of the city."
 * The intro to Left 4 Dead features this exchange:


 * Mass Effect 1 ends with an epic battle against the Reaper known as Sovereign, which is only narrowly defeated by the combined efforts of the Citadel and Terran fleets. However, Sovereign is only a vanguard, left behind by the Reapers to secure their return. There are more where he came from, and they're coming back one way or another...
 * At the end of Mass Effect 2 over 200 Reapers have been counted. Just one nearly wiped out the Citadel Fleet.
 * And in the trailer for Mass Effect 3,
 * Happens in Sonic Chronicles. The first Marauder that Sonic encounters is only a scout. Sonic expects an easy fight, only for the scout to turn out to be a difficult boss (well, difficult for a first level boss).
 * The first few battles against the Shivans in Free Space are completely one-sided, with the Shivan ships being Nigh Invulnerable, extremely maneuverable, and heavy-hitting in damage. A few upgrades later, the player can fight these monstrosities evenly, and even manages, with a huge amount of effort, to capture what they think is a Shivan command ship. Then it turns out that those were outdated Shivan scouts, and the "command ship" was among the weakest deployed Shivan warships... the actual muscle of the Shivan armada soon arrives and pretty much steamrolls the Terran and Vasudan fleets, no matter what countermeasures they come up with.
 * Also invoked by the tagline of the second game: "The Shivans are back, and they're wondering what happened to their scouting party."

Web Comics

 * Used in this page of Looking for Group.

Web Original

 * The Rooster Teeth series supreme surrender is based around this trope.

Western Animation

 * Transformers Prime in Crossfire now wanted by Megatron, controls an Insecticon to get her revenge. The Insecticon really got Meg's on the ropes, but after he defeated it, she realizes that it was only a scout, as she found an entire nest of Insecticons.