Theatre/Opera/Ballet Buffs?

Well, there's always the "Only a miracle can save us now!" scene from History of the World, Part 1 where the white horse miraculously shows up and...

not buying it, huh?

Okay, how about in The Lord of the Flies where the captain shows up? Or at the end of Shakespeare's A Winter's Tale where the statue miraculously becomes animated and is the real Hermione?

Ooh! How about Shakespeare's Romeo and Juliet, where Romeo runs in at the end and... oh, wait. That one ends badly.

Okay, how about Aida, where at the end the lovers are in the cave and... darn! That doesn't end well, either.

Madame Butterfly? Othello? Julius Caesar? Symphonie Fantastique? Faust? Oh, dear, I'm no help! All the ones off the top of my head are tragedies. Sorry!

The only ones I can think of with a happy ending are more modern than you want to go: The Nutcracker, Peter and the Wolf, the Magic Flute, and maybe The Taming of the Shrew?

The Nutcracker ... the battle scene

Most operas end tragically. There are a few “opera buffa” and Rossini’s comedies.
Look up: Barber of Seville, La Cenerentola, but probably William Tell is what you want…. it has your ending.

This link might help: http://www.theatredatabase.com/medieval/medieval_theatre_001.html. They have a bookstore where you can buy the play and read it. This has some additional links: http://www.theatrehistory.com/medieval/ (You want to look at morality plays--those were the ones that had battles).

You also might want to research theatre history, since the time frame you're referencing is an important era in theatre. If you can get access to a local college with a good theatre department, they will have excellent resources. Don't forget to ask your librarian, too!

Try the Tv Tropes page for Deux Ex Machina. Several choices there, depending on how you define cavalry.

Hmmmmm...

There is the climactic scene in Tartuffe (by Moliere) in which the King's officers recognize the bad guy after he has successfully swindled an entire family. Thus he is arrested, the victimized family restored, and a lesson learned (not least that the King is a wise and just sovereign).

The last scene in Shakespeare's Measure for Measure in which the supposed trouble-making Monk brought for judgment before Angelo, the Duke's regent, turns out to be none other than the Duke himself! While the young man "executed" it is revealed was not and is still alive to marry his lady love.

Many Greek tragedies end with a deus ex machina (that is where the term comes from, after all) including Medea.

At the climax of HMS Pinafore, the sudden revelation that the two main male characters were switched at birth resolves virtually every single detail and conflict of the plot (even though it makes no sense whatsoever).

There's the opera Hansel and Gretel.
http://www.metoperafamily.org/metopera/history/stories/synopsis.aspx?id=48

Lots of Sturm und Drang plus a happy ending, unless you're the witch. I've seen it and I think it'd work for your needs. It's a big, long opera (2 1/2 hours??) and time for your hero/army to show up and save the day.

It's kinda modern, but The Threepenny Opera ends with a messenger from the king pardoning the protagonist is just going to be hanged and giving him a pension and a castle. Of course, this is a parody of this typical convention.

Phantom of the Opera. The Phantom forces Christine to come with him to save the Viscount. Then the Phantom finally shows remorse, and turns her loose. She escapes the catacombs under the theater with her true love, the Viscount.

The book ends much better. Christine voluntarily spends the night with Eric (the Phantom), escapes and marries the Viscount, immediately becomes pregnant, and has a beautiful, talented, genius boy, who looks nothing like the Viscount.

There's one famous opera that has a nick of time rescue: Beethoven's Fidelio wherein the hero is imprisoned unjustly, and the evil jail warden is going to kill him and bury him because the government inspector is coming. Our hero's beloved Leonora disguises herself as a man, becomes a prison guard, and holds the jailer off with a pistol until the inspector gets there, discovers the warden's evil doings, and arrests him.