Trapped Behind Enemy Lines

An elite unit of soldiers is cut off behind enemy lines and has to fight their way to safety. It's usually the result of hubris either on the part of the unit or (more often) their higher command.

Common causes:

An attack went too far, or didn't get sufficient support from other troops, and the enemy cut off their line of retreat.
Did not retreat quickly enough from an overwhelming attack, and the enemy cut off their line of retreat.
Transport failure dumped them in the middle of nowhere.
A raid went awry, cutting off their escape route.
Prisoners of war escaped.
In the case of mercenaries (like Xenophon), the sudden loss of an employer can do this as well.

May lead to a You Are in Command Now situation. Usually contains at least some Field Promotion. You Shall Not Pass may be required—or worse, a Last Stand.

Film

Bat*21
Behind Enemy Lines and its sequels.
Black Hawk Down - Based on Real Life.
A Bridge Too Far - Based on Real Life.
Centurion
Flight of the Intruder, both the book and the movie.
The Warriors
The Wild Geese (1978) and its remake (2010)

Literature

Xenophon's Anabasis (on which The Warriors is loosely based), making this at least Older Than Feudalism. The alternate English title for the book provides the trope's name.
- Also Andre Norton's novel Star Guard (1955)
- Also Harold Coyle's novel The Ten Thousand (1993)
- Also David Weber and John Ringo's Prince Roger series of novels, beginning with March Upcountry (2001)
Happens in just about every Sven Hassel novel, including an obligatory scene where Sven and his colleagues find themselves in the midst of enemy troops and either get mistaken for a special unit (e.g. Volga Germans or Foreign Legion), or saved by Porta's quick-thinking chatter.
The Ciaphas Cain novel Death or Glory starts with the titular protagonist and his faithful aide Jurgen landing in the middle of an Ork-infested continent, half a world away from the front lines. In Cain's attempts to find friendly forces to put between himself and danger, things snowball until he's cobbled together a small army of soldiers, civilians and militia cutting its way out from the heart of Orkish territory and turning the tide of the whole war.
At the end of the Wing Commander novel Action Stations, Max Krueger's ship is shot down over a Kilrathi-held human world during a raid on Kilrathi assets in the area.
Honor Harrington's fate after the break-out from the State Sec ship Tepes, at the end of In Enemy Hands. Followed in the next book by probably one of the biggest prison breaks in all of history, fictional or Real Life, escaping with roughly half a million prisoners.

Live Action TV

One episode of Star Trek: Deep Space Nine has Nog and Jake in a shuttle, inside Dominion space, about to be blown up... and then the Valiant, which has been trapped in Dominion space since the start of the war (because of their captain's ego more than anything else, it's a crew of cadets, one of whom was given a field promotion to acting captain and ignored the common sense/orders to return home), shows up and rescues them. The rest of the episode deals with their self-designed mission behind enemy lines.
The Polish WW 2 series "Czterej pancerni i pies" had this as a recurring plot device. The titular tank crew would find themselves behind enemy lines either because an enemy counterattack managed to cut off their position or because their mission was to move around the enemy and attack them from behind.

Newspaper Comics

In Peanuts, this happened to Snoopy a lot when he was pretending to be a World War I Flying Ace.

Video Games

Mechwarrior 3
The last mission of Halo: Reach is this. The player character is the last surviving member of Noble team on Reach. The mission is a desperate Last Stand.
Shadow Company: Left For Dead begins with your employer abandoning your mercenary group in the middle of enemy territory after a mission goes awry.
Subverted by Tales of the Abyss. It could've been played this way, but Tear uses her "I'm from the Order of Lorelei" card, which ropes Jade (who should've been skewering Luke since their nations are at war) into helping out instead. (And Jade's reasonable enough to ask some questions first anyhow.)
Free Space 2 has a bonus mission that drops a small group of starfighters at an unknown jumpgate deep inside enemy space to explore where it leads to record anything they can. The extraction ship drop from hyperspace exactly 15 minutes later and leave immediately if the pilots are not back by then.
Call of Duty 4: Modern Warfare has the missions "Hunted" and "Death from Above" where the player's SAS Squad must fight to the evacuation point after their helicopter is shot down. Ironically, they were heading home after rescuing someone else from enemy hands. The missions "Heat" and "One Shot, One Kill" also count.
- Modern Warfare 2 does this with the Escape from the favelas mission.
The main story of Valkyria Chronicles has this happen to Welkin and Alicia after a mortar causes them to fall of a cliff. One of the DLCs has this happen to Edy Nelson and some of the squad after she goes Leeroy Jenkins on some Imperials and Marina gets distracted by a puppy, pulling them even further in.
Operation Flashpoint : And how ! Nearly in every other mission of the campaign...
Most of Silent Hunter series is spent in enemy waters as that is a submariner's job. And of course you can easily get trapped there.

Tabletop Games

Diplomacy, a pre-WWI simulator (sort of): subversion. If this happens to one of your units, it can actually strengthen your position.

Web Original

Happens quite a bit in the first book of The Salvation War, seeing as everyone who dies ends up in Hell. Leads to the formation of La Résistance, with the odd historical character or twelve joining, no less.
A common enough thing on any wargame naturally.

Real Life

The 1st Marine Division at the Chosin Reservoir during the Korean War.
The Czech Legions escape from Siberia during the Russian Civil War
- And on a smaller scale, other RCW operations and escapes, including the Ice March and the evacuation of various allied contingents at the war's end.
The Battle of Mogadishu, which was the direct basis of the aforementioned Black Hawk Down.
Xenophon and the 'Ten Thousand' after the battle of Cunaxa.
World War II's large-scale airborne invasion Operation Market Garden. It pretty much caused twice the losses to the Allies than the Axis.
To a (somewhat) lesser degree, the airborne landings during the Battle of Normandy. Due to the transport pilots attempting to avoid anti-aircraft fire (or sometimes failing to avoid it), Allied paratroopers were scattered all over the French countryside, lost and confused. The fact that they were able to rally and improvise (and generally cause masive confusion to the Germans trying to figure out just what the equally confused paratroopers were trying to accomplish), led to the military concept of Little Groups of Paratroopers (LGOPs for short), describing the resulting units as "A bunch of nineteen year olds, poorly supervised, pissed off, armed to the teeth, and vaguely recalling their orders as "Shoot anyone who is dressed differently."" Those troops that managed to link up with Allied forces moving inland from the beaches were able to pass on valuable intel, in addition to delaying German counter-attacks against the beachheads.
Several of the Wehrmacht's 'moving pocket' operations on the Russian Front.
The notorious Retreat from Moscow, in 1812.
During the Boxer Rebellion the foreign residencies in China stood siege for several weeks defended by a coalition of their respective garrisons.
This is the basis of the 28th stratagem. The consequence is that, should the affected enemy successfully clear your forces and the elements, he will make absolutely positively sure he is not alone the next time he has to deal with you.