Binding Ancient Treaty

Generations ago, the leadership of your ancestors were great friends with the people of Ruritania. They got along great and scribbled up all sorts of trade and defense agreements.

That was then. You guys haven't been that good of friends since that time, but now that The Empire is knocking on Ruritania's door and massing troops at its borders, this crapsack country's king has sent his ambassadors to their dilapidated embassy in your capitol with the message, "You guys signed this treaty, you need to come help us!"

They've got you. Never mind the fact that nobody's cared about or invoked that treaty for hundreds of years, or the fact that every mortal involved with its writing is dead now. Your (country's) word is your bond, right? Not that you ever wanted to go up against The Empire; it's probably going to bring ruin to your nation... but, at least you have your honor.

The compulsion to honor the treaty might be magical; especially if the person that squeezed the treaty out of your country is an immortal sorcerer of some sort, or it might be based on a country's integrity or international legal code... There could be some penalty even worse than war if you renege on the terms of the treaty.

Particularly useful in enlisting the aid of a Proud Warrior Race Guy. Gondor Calls for Aid often results from one of these treaties.

Examples of Binding Ancient Treaty include:

Card Games

Happened on the plane of Ravnica in Magic: The Gathering, where a supernatural treaty named the Guildpact was signed to bring peace to ten warring factions.

Comic Books

Used multiple times in Uncle Scrooge comics, with various factions competing to obtain symbolic artifacts that, due to ancient land ownership treaties, cede control over huge stretches of land to whoever owns the artifacts. In the end, when this results in the Native Americans owning Europe, governments are finally spurred to take action to stop this from happening again.
- It should be noted that the comic itself points out why this is hypocritical behavior, with Donald and his nephews bringing up this fact to bring an end to the whole circus.
One Simpsons comic invokes this trope when the Simpsons end up living in a mall (Homer refused to sell the house to the development corporation responsible for turning the neighbourhood into said mall, so they built it around the house). Lisa does some research in order to get the corporation to tear down the mall, and discovers that the founder of Springfield signed a peace treaty with some Native Americans who had saved him from the brink of death, giving them sovereignty over their land (which happens to be the land the mall is on), only to betray that treaty later on by burning their village to the ground and killing all he found. Luckily, one member of the tribe escaped with her baby, and the treaty states that the land belongs to any living member or descendant of the tribe, which in this case happens to be Lenny.

Film

In The Lord of the Rings: The Return of the King, Gondor Calls for Aid, and Rohan answers. Leads to a very cool scene where signal fires on various mountain tops, maintained by likely very cold soldiers for hundreds of years, are lit in succession to tell the Rohirrim that Minas Tirith needs help.
It seems like the humans in Hellboy II: The Golden Army really ought to have remembered and honored their treaty with the elf king to leave the forests (and the supernatural creatures that live there) alone. Of course, that treaty was written hundreds of years ago. As it stood, they risked an unstoppable army of magic robots descending upon the human world.
The Canadian comedy Buried on Sunday centered on a small fishing community on an island of nova Scotia that due to a provision in an old treaty between Canada and Denmark had the right to secede from Canada. When their fishing quota is taken away the village invokes the treaty and declares themselves their own country. The Canadian government considers this a joke until they find out that a damaged Russian nuclear submarine came to the island and its four remaining crewmen sold the sub with its missiles to the villagers. They now have to take the treaty seriously since the village is now a nuclear power.

Literature

In The Lord of the Rings, the Rohirrim have been living for five centuries in an ex-province of Gondor in return for aid in war.
The Treaty of Vo Mimbre in The Belgariad established a bunch of things that turned out to be very, very useful for the resolution of the plot. Including getting the main character married. Needless to say, the instant it gets brought up, the Tolnedrans immediately try to alter it on the grounds that, well, it's hundreds of years old. (The protagonist also makes a couple of alterations on the grounds that he wants to be happily married.)
A strange version of this is attempted in the Judith Krantz novel Dazzle. While trying to prevent her father's land from being sold, Jazz digs up an ancient covenant signed in the 1800's saying that the parties involved wouldn't sell the land. Since this book takes place in the 1990's, that doesn't mean a darned thing when you are trying to prevent the sale of a ginormous amount of untouched land in southern California.
In Star Trek: Ex Machina, the Shesshran race had one of these with the Fabrini, which they honoured when the Fabrini's Yonadi descendants later wanted to colonize a world in their star system. In Shesshran culture, contracts and promises are held in the highest esteem, so even though they weren't entirely happy about it, they were quick to permit the Yonadi settlement on the neighbouring planet.
Inverted in the Heralds of Valdemar novel By the Sword, when Valdemar comes to Rethwellan to beg for military aid against Hardorn. Rethwellen's king is aware of a treaty from a few generations back that gives Valdemar the right to demand their help, but Valdemar seems to have forgotten about it, and he isn't much inclined to enlighten them. It falls to the protagonist, who is the granddaughter of one of the witnesses to the agreement, to bring everything to light and secure the necessary aid, which turns out to be in the best interests of everyone after all.

Live Action Television

Aversion in Andromeda. An alien race is shooting at Dylan's ship and causing trouble for the people he is trying to convince to join his New Commonwealth when Dylan broadcasts some code. Beka is surprised and incensed that he's trying to talk with these people, only for them to stop shooting, but then the aliens (known as the Pyrians) proceed to say how the treaty Dylan is invoking was considered null and void with the fall of the original Commonwealth. However, they are willing to talk at this point.

Real Life

While the network of interlocking treaties that triggered the massive involvement in World War I largely only went back fifty years, that's still a long enough time for the actual signatures of the people who scribbled the treaty to not be the same as those who were in charge at the start of the war. Given how devastating it was for everyone's chits to be called in at once, WWI deserves special mention here.
- Of particular note is the treaty which brought Britain into the conflict: the Treaty of London (signed 1839), which was so archaic that even after being told it was being invoked the German Chancellor could not believe the two countries were going to war over "A scrap of Paper".
- It's entirely possible (and theorized by more than a few historical scholars) that the "ancient alliance treaties" were just a good way to get involved. Germany wanted France and a large part of Northern Europe. England had a vested economical interest in seeing the balance of power in Europe stay the same. Everyone acted as they wanted and justified by saying they needed to.
The Portuguese invoked their alliance with England, written in 1373, during the Portuguese-Indian War in 1961. Great Britain was bound to offer "troops, archers, slingers, galleys sufficiently armed for war."
The treaties arising out of the Peace of Westphalia (1648) after the Thirty Years' War delineated the makeup of Europe's national boundaries for a rather long time. That's not the important and still binding part. The important parts are:
- The specific endorsement of National Sovereignty, meaning that a country cannot be interfered with in its internal affairs (the fact that something was going on internally in another country was often used as an excuse to start a war), and:
- An explicit universal prohibition against Piracy, meaning that any country could try any pirate in any court anywhere in the world. Both concepts are still in force today throughout the world.
Britain still has a right to recruit in Nepal, and uses it. It was included in the peace treaty of a war long ago at the request of officers who had seen the Gurkhas as enemies and were so impressed that they wanted them as allies.
The treaty that created NATO was signed over sixty years ago. The threat for which it was originally designed- the Soviet Union and its communist puppet regimes- has not existed for over twenty years. Article Five of the treaty states that an attack on one member (with a couple of geographic caveats to keep territories and colonies from being a trigger) is equivalent to an attack on all members, meaning that if some minor member of NATO gets into a shooting war with a major world power then it could ignite World War Three. On the other hand, that very fact has likely prevented several conflicts from escalating further, and it has found new life as an organizing force when Europeans and/or the US get involved in overseas conflicts such as Libya.
Brutally subverted in Thucydides telling of the Peloponnesian War. The Athenians wanted the island of Melos. The Melians wanted to be independent. Athens told them that they will submit, or be invaded and crushed. The Melians appealed to Sparta (which Athens was in a sort of Cold War with), with the central argument that Sparta had such a treaty with them. Sparta told them that such a treaty meant nothing. The Melians held out anyway. Athens invaded, and wiped out the Melians.

Video Games

Be careful about who you make a permanent alliance with in Civilization 4, because you'll be going to war with whomever that faction fights.
In Dragon Age: Origins, the plot revolves around the Grey Wardens using a set of Ancient Treaties in order to rally the Circle of Magi, the Dwarves of Orzammar and the Dalish Elves to create one Badass Army in order to take on the Archdemon and end the Fifth Blight.
Subverted in Homeworld: When the Kushan test the Hyperdrive for the first time, they unknowingly broke a 4,000 year old treaty forbidding them to do so. This results in the Taiidan Empire committing genocide on the Kushan.
Same in Master of Orion 2. Sign an alliance with one race, and they'll ask you to declare war next month. If you refuse, they declare war on you. Just maintain non-aggression pacts and remain neutral while the other empires kill each other off.
Naturally, these crop up on occasion in Cyber Nations. Usually the signatories downgrade a treaty if they're that out of touch with each other.