Overnight Conquest

In Real Life, wars are a messy and tiresome affair. Hostilities can escalate for months long before the sides even exchange the first shots. Even the best war machine in the world will still take its time securing the enemy lands, trench by trench, if not because of fierce resistance on both sides of the frontlines, then because marching in too fast will stretch the lines of communications and supply too thin for normal operations.

In fiction, however, the dramatic rules require the invaders to appear larger than life, to establish them as a credible threat unbeatable by normal means. The simplest way to emphasize the superiority of the invader's military might and technology is to establish that they completed their takeover before the invaded state even knew they were there. The survivors will later refer to the invasion as the "One Week War", or the like.

Alien Invasions often receive this treatment but it is not limited to them; for instance, rogue AIs usually take nanoseconds of processing time to peg humans as the enemy and launch a Robot War. However, it is almost always some fantastic element that allows for this trope to occur and bypass the standard logistical problems.

A large-scale subtrope of Curb Stomp Battle and, in many cases, Easy Logistics. A common setup for Back From the Brink scenario. Home by Christmas would be when someone is expecting this kind of war, only for things to turn out much less than expected. Compare also Easily-Conquered World, wherein the defenders fail to offer any resistance to much weaker invaders.

Note that examples do not need to take place literally overnight; any improbably-quick conquest (relative to the size of captured territory) qualifies.

Anime and Manga

 * This is pretty how the Kushan Empire invaded the Capital City of Windham in Berserk. Moments after the King of Midland dies, Emperor Ganishka and his troops come storming in and take over the joint.

Comic Books

 * Marvel Comics had a 2012 Crisis Crossover called It's Coming which dealt with a Bad Future in which The Phoenix came back to Earth and reduced it to an ash-filled wasteland devoid of life. In that future, they called the cataclysmic event "The Six-Second War".

Film

 * In Canadian Bacon the Canadian-American war ends before hostilities can even begin when the Americans realize that all their nuclear weapons can be remotely controlled from Toronto. The American President surrenders to the Canadian Prime Minister who is not even aware that anything out of the ordinary is happening.

Literature

 * Robert Heinlein's novel Sixth Column (AKA The Day After Tomorrow). The PanAsians use their vortex beams and A-bomb rockets to defeat the United States in less than a day.
 * The Mouse That Roared: The micronation of Grand Fenwick declares war on the US with the premise that they will inevitably lose and then get war reparations, pumping money into the moribund Fenwickian economy. Then they accidentally capture the Q-Bomb and win. This is before anyone in the US even knows the war is going on.
 * At the beginning of The Ellimist Chronicles, Toomin and his friend are playing a war game. Toomin's side is defeated so quickly he has to watch the replay in slow motion to find out what happened.
 * Cain's Last Stand makes a very big deal that an invading Chaos force is progressing far faster than it has any right to, and even seems to be growing. They find out that it's because Warmaster Varan has . In addition, Necrons have weapons that desintegrate targets and can teleport to just about anywhere they please, resulting in a Curb Stomp Battle.
 * The prequel book of The Tripods indicates it only took a few weeks after the Trippies started passing out Caps for the world to end up with more Capped than Uncapped. Some pockets of fighting went on for a while, but it's obvious by the end of the prequel that the Capped are in control.

Live Action TV

 * The reboot of Battlestar Galactica begins with the near-obliteration of the human race in a surprise attack that's over before anyone's realized what's going on.

Video Games

 * Between the first incursion in Half Life and the thoroughly subjugated world of Half Life 2 is "The Seven-Hour War".
 * The Psychlos of Battlefield Earth beat down humanity in about nine hours. Presumably all the mighty warriors responsible have since left to conquer other galaxies and left the dregs on this backwater; it's the only possible explanation.
 * In the Strangereal continuity of the Ace Combat series, the Belkan war began on March 25, 1995 and would have ended with Belka's victory on April 2, if it were not for the single remaining ace (that's you) who delivered his country Back From the Brink.