War Was Beginning

In A.D. 2101 (Beat)
War Was Beginning.

—Zero Wing

In any film beginning in wartime In Medias Res, it is almost taken for granted that a narration intoning opening text will describe the start of the war and provide a little history lesson... even if it's a fictional war.

Opening narration / text crawl, often over a map, with drums or other fanfare. (This trope is big on drums.)
- Story begins In Medias Res...
- Usually, with an Action Prologue.

Opting for an Action Prologue with narration on top, over stock footage of combat or Stuff Blowing Up may produce Narm: People don't generally like to mix their opening crawl with their Opening Action Sequence.

However, one may immediately cut to combat scenes In Medias Res as soon as the narration ends; indeed, it's almost expected, to the point where you can throw people off (but not really) with quiet scenes set in a better time/place, if you're seeking a slow build:

Cut to a quiet scene in Arcadia; the hero will learn of the impending war about to wreck his world by way of a Herald, such as a wounded relative; see Bearer of Bad News, Harbinger of Impending Doom.
Or the hero may turn out to be tragically related to the prosecution of the war somehow, such as: a family member started it, or got killed in the initial attack.

This opening narration requirement can be removed if, as often the case, the writer decides on one of the alternatives, such as:

Show how the future protagonists react to the start of the war, in a "where were you when"... way. This is a useful way to introduce the future protagonists to the audience one after the other—a Debut Queue.
Depict the events leading up to the start of the war, even if it happened years before the main story. This will be done in an Opening Action Sequence involving Stuff Blowing Up, possibly Earth.
If it's a war "everyone's heard of", the audience viewpoint character is simply informed that a war is on, averting this trope.

Except perhaps with a newspaper headline (anachronistically titled) "World War Two Begins," or worse, "World War I Begins," as if people back then knew it would be the first of two.
Surprisingly, #3 is less common, due to Small Reference Pools: World War II movies filmed during the war regularly opened with a War Was Beginning recap.

Not to be confused with During the War.

Anime and Manga

Howl's Moving Castle may be an example. There is no narration explicitly telling us that war is coming, but soon after the opening of the film we see a big parade of troops, tanks, and flying machines moving through the city to cheering crowds, and we overhear conversations on the street about rising tensions with a neighbouring kingdom over a missing prince.
Mobile Suit Gundam opens with a description of the events of the One Year War before the series begins.
The very first thing Neon Genesis Evangelion shows us (after the great intro) is: "In the year 2015 AD". Cue peaceful landscapes full of military hardware. Cue giant monster...
Code Geass establishes the general rise to power of the Britannian Empire (more specifically their invasion of Japan) in the first episode.

Film

Narration

Lord of the Rings may be the Trope Codifier.
Casablanca: "With the coming of the Second World War..."
To Be or Not to Be: "Lubinski... Kubinski... Lominski... Rozanski and Poznanski... we're in Warsaw, the capital of Poland. It's August, 1939. Europe is still at peace..."
300—combination of an Arcadia opening and How We Got Here, with a dark Persian Herald descending on the golden plains of Sparta. Somewhat subverted in that the entire movie is narrated with ominous "and so it begins" intonation by David Wenham.
Mad Max: "For reasons long forgotten, two mighty warrior tribes went to war and touched off a blaze which engulfed them all."

Text

Every Star Wars movie, although Revenge of the Sith is the most direct about it, as it begins its Opening Crawl with the phrase: "War!"
- It is a period of civil war. Rebel starships, striking from a hidden base, have won their first victory against the evil Galactic Empire...
The opening of Troy (a particularly wordy example), and...
Last of the Mohicans. Both with drums. Always with the drums!

America 1757
It is the third year of war between France and England for possession of the continent.
Three men, the last of a vanishing people, are on the frontier west of the Hudson river...

The Adventures of Robin Hood: "In the year of Our Lord 1191, when Richard the Lion-Heart set forth to drive the infidels from the Holy Land..."
- Robin Hood (2010 film) does it similarly.
Fire Over England begins with a remarkably pompous and tendentious one:

"In 1587 Spain Powerful in the Old World Master in the New Its King Philip Rules By Force and Fear But Spanish Tyranny is Challenged by the Free People of a Little Island England..."

Master and Commander.

Live-Action TV

Parodied in the Monty Python's Flying Circus "Ypres 1914" sketch. This was used as a Cold Open in several Monty Python sketches.

In 1914, the balance of power lay in ruins. Europe was plunged into bloody conflict. Nation fought nation. But no nation fought nation morely than the English hip hip hooray! Nice, nice yah boo. Phillips is a German and he have my pen...

The first Horatio Hornblower TV film took place at the Royal Navy anchorage in Spithead, with the midshipmen discussing the ongoing revolution in France. In particular, the fact that King Louis XVI had just been captured.

What do you think they'll do with him? You can't kill a king!

Falling Skies just pulled off this trope with a little kid in the role of narrator, recounting what he knows of the alien invasion to date while his crayon drawings are displayed.
Star Trek: Voyager began with an Opening Scroll briefly detailing the Maquis guerrilla war against the Federation and Cardassians, then cut to Chakotay's ship, involved in a running lightfight with a Cardassian destroyer. The whole sequence looks like a Shout-Out to A New Hope, except Chakotay's ship escapes.

Video Games

The Trope Namer is a quote from Zero Wing; that is an example as well.
- More Narm-fully, it features a cold open over Stuff Blowing Up, then the narration starts.
Also, Dune II: The Building of a Dynasty. "Vast armies have arrived. Now, three Houses fight for control of Dune. (sic) Only one House will prevail. Your battle for Dune begins... now-ow-ow-ow-ow..."
Yggdra Union and Yggdra Unison both start with the classic text scroll as Gulcasa's attack on Karona is related to the player.
The first part from the real time tactical Myth game series also begins like this: "In a time long past, the armies of the Dark came again into the lands of men."
Super Metroid: "The galaxy is at peace."
Every game in the Escape Velocity series starts by explaining the current war. The original and Override had opening scrolls (detailing the Alien War and the start of the Rebellion in the first case, and the United Earth/Voinian War in the second). EV Nova changed it to four splashscreens.
X3: Terran Conflict explains the roots of the titular Space Cold War in its opening cinematic. X3: Albion Prelude shows us how that cold war turned into a hot war.
Fire Emblem: The Sacred Stones starts with this, then segues into The Empire sacking the protagonists' castle.
Asura's Wrath does this for it's opening level.. It is awesome.

"The foul, impure beasts that roam the land ~ the Gohma.
In order to save civilization from their neverending threat, the demigods who governed the world created an ultimate weapon that would wipe the Gohma of the face of the planet once and for all.
Their legions were led by eight elite demigods ~ the Eight Guardian Generals. One of them, General Asura, was in the thick of battle."

Mass Effect 3 opens with a text crawl that recaps the events of the first two games and Arrival:

In 2157, humanity discovered that it was not alone in the universe.
Thirty years later, they found a peaceful place among dozens of galactic species.
But this idyllic future is overshadowed by a dark past: Reapers, a sentient race of machines responsible for cleansing the galaxy of all organic life every 50,000 years, are about to return.
The leaders of the galaxy are paralyzed by indecision, unable to accept the legend of the Reapers as fact.
But one soldier has seen the legend come to life.
And now the fate of the galaxy depends on him.

- ...then enters a fairly quiet scene involving the Player Character on trial for the events of the latter—which is interrupted by a full-blown Alien Invasion.

Western Animation

Avatar: The Last Airbender's Title Sequence has a Katara voiceover briefly describing the Fire Nation's war and the Avatar's failure to stop it. The first episode goes into greater detail on her people in particular. The first few episodes involve giving Aang a history lesson on the world war he slept through.