Altar Diplomacy

"Let others wage war, but thou, O happy Austria, marry; for those kingdoms which Mars gives to others, Venus gives to thee."

Not pictured: when Camilla told Ryoma "KISS ME ALREADY, YOU FOOL!"

Arranged Marriages are already guided less for love (not always, however) and more for interests coming from the families or clans involved. This trope takes it Up to Eleven: the marriage in itself will seal a peace or an alliance between either nations, smaller lands, companies, businesses, etc. All sides in this deal will reap social, political, economic, etc. benefits - though it's VERY likely that the spouses won't be too happy: either the Rebellious Princess / Rebel Prince will try to run off, The Dutiful Son will play the role with clenched teeth, the local Authority Figure (maybe The High Queen, The Evil Prince, The Wise Prince, etc.) will personally offer themself as a spouse for the sake of their lands, or The Conqueror will force a royal from a land they've adquired into this to make their conquest 100% legitimate...

This has been Truth in Television through near all eras of history, since marrying solely out of love is a relatively new idea. 

Examples of Altar Diplomacy include:

Anime and Manga

• In Hetalia, Poland and Lithuania meet for the first time when Liet's boss (Great Duke Wladislaw Jagiello II) and Poland's boss (Queen Hedwig/Jadwiga of Poland) decide to pull this to start the Poland-Lithuanian Union.
• In Sengoku no Mikazuki, before the war between the Kitakata and Nagumo lands began, their lords had made a peace treaty and one of the deals in it included marrying off Princess Natsu, the daughter of the Kitakata Lord, to the Lord of Nagumo, Iwano. But Iwano broke the treaty, had the whole Kitakata clan killed save for poor Natsu, and re-started the hostilities...
• Zettai Heiwa Daisakusen has Prince Johane and Princess Euda invoking this trope to stop the war between their lands. But they don't ultra get along.
• In a way, what happens in Nisekoi: despite NOT getting along, the Mafia Princess Chitoge and the Yakuza Prince Raiku decide to start fake-dating to keep their Feuding Families from going to an all-out gang war. . .

Films

• Ever After has Prince Henry of France engaged to Princess Gabriela of Spain, even when Henry is still hung up on Danielle and Gabriela is in love with the Spanish retainer Pepito. The marriage itself is about to fully take place, with poor Gabriela sobbing through it, until Henry decides to stop the charade and first lets Gabriela go to Pepito, then goes after Danielle.

Video Games

• Fire Emblem:
  • In the Archanea games, Prince Hardin of Aurelis and Princess Nyna of Archanea's marriage was this. Hardin had a Bodyguard Crush on Nyna, but while she thought well of him, she married him for political reasons (and was still in love with Camus). Plus the other guy with enough Blue Blood to be her husband was Prince Marth of Altea, and Nyna already shipped him quite a bit with Princess Caeda of Talys...
  • Can be done in-game in Fire Emblem Fates - more exactly, in the Revelations route. A Prince / Princess of Hoshido can perfectly marry a Prince / Princess of Nohr in this Golden Path, making the truce between both warring kingdoms potentially stronger... and it's even better if either Azura (the Fallen Princess of Valla) or an Avatar of either gender (who's crowned as King or Queen of Valla at the end), are included. The ones pictured above are Ryoma, the Crown Prince of Hoshido, and Princess Camilla of Nohr, one of the younger sisters of Crown Prince Xander. As a bonus, since both countries have more or less joined an alliance rather than fighting one another (unlike the other two paths), the supports between the candidates make such unions double as Perfectly Arranged Marriages.
  • f Petra Macneary and Hubert von Vestra from Fire Emblem: Three Houses reach an A support (only possible in the Crimson Flower path, since Hubert cannot be recruited away from the Black Eagles House), their shared ending mentions rumors about Hubert's lady in liege Emperor Edelgard telling them to marry so they'd forge an strong alliance between the Adrestian Empire and Petra's homeland of Brigid. It also adds that busy as they were, they dearly loved one another and had many children.
• This can be done in Dragon Age: Origins, specifically during the Landsmeet. Three prospect results can invoke the trope: marrying the legitimate heir Alistair (the half-brother of the late King Cailan) to the Queen Dowager Anora (Cailan's widow), having a Female Noble Warden marry Alistair, or having the Noble Male Warden marry Anora..

Real Life

• As said above, this was the NORM among nobles and rich people in older times.
• The quote above was provided by Maximilian I of Habsburg, Holy Roman Emperor from 1508 to 1519, and logically quite the believer in the trope. He himself followed it via marrying Duchess Mary of Burgundy and then having his son Philip I of Castile (aka Philip the Handsome) marry Joanna I of Castile (aka Joanna the Mad). His descendants, the House of Habsburg, would turn this trope Up to Eleven through the centuries.