erm, kinda bumping this thread up.
I'm in the mood for typing long stuff.
Exa, feel free to correct some of my details.
Long story short:
3,000 years pass between the Prologue and the War of the Ring (Ring Quest), 3,019 years to be exact.
Here's how it goes: remember those various references to "Numenor" and the "Numenorians"? (i.e. "the blood of Numenor is now spent; it's pride and dignity forgotten"
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***Numenor was an island nation of Men; it is the "Atlantis of Middle-earth"--> this is more than just a small connection; the ELVISH (Quenya) name for Numenor is "Atalante", making it an obvious parallel to Numenor.
Anyway, remember that the Second Age ends and the Third Age begins with the first downfall of Sauron, as seen in the Prologue.
During the Second Age, about 100 years before it ended or so, Numenor sank into the Sea in a catacylsm (it's very complicated, but that will do).
The survivors were those who heeded warnings of disaster early, and were out on ships when it happened. The great storm that resulted blew their ships to Middle-earth, and they founded the "kingdoms in exile" (so called because they were 'exiled' now that Numenor was destroyed).
**The surviving Numenoreans that arrived in Middle-earth were now called the Dunedain (from Elvish; Dun=west, and edain=men, thus Dunedain=Men of the West, because Numenor was in the middle of the great ocean to the WEST of Middle-earth).
These first survivors are the ones that fight Sauron, but their descendants are just called Dunedain. You may remember this from some scenes where they talk about Aragorn being a Dunedain Ranger. Aragorn being one of their descendants.
***The leader of the survivors was Elendil: he's the King that Sauron kills in the Prologue.
Elendil's TWO sons were Isildur and Anarion, and the three of them led the surviving Dunedain.
They founded *TWO* kingdoms for the Dunedain in Middle-earth, along the coast: Arnor in the north (the Shire was part of it) and Gondor in the south.
Elendil was the High-King of the United Kingdom of Arnor and Gondor; both were one big country.
Elendil also directly ruled Arnor, while Isildur and Anarion jointly ruled Gondor.
***During the War of the Last Alliance against Sauron (in the Prologue) Anarion died in battle before the final battle, so he dies before the movie starts. Then Elendil dies, so Isildur is the last High-king.
*****Here's what the movies don't really explain: Anarion had a son, Meneldil.
Isildur decided that he still couldn't directly rule everything, so he decided that he would ride back north to rule Arnor, while leaving Meneldil as King of Gondor.
Problem was, on his way back north, Isildur and his men were ambushed (as seen in film) during the Disaster of the Gladden Fields. Isildur himself had four sons, but three were with him during the ambush so they died too. His only living son was the young Valandil, who at the time was in Rivendell.
So Valandil son of Isildur became King of Arnor, and Meneldil son of Anarion was King of Gondor. They were first cousins. But while they were never hostile to each other, King Meneldil said he didn't think Valandil should be king of both countries; so Arnor and Gondor split into two separate countries.
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So for the next roughly 2,000 years, the Heirs of Isildur ruled Arnor, and the Heirs of Anarion ruled Gondor; ***The catch is that Isildur and Anarion were brothers, and that before the split their faither Elendil ruled a united kingdom.
Arnor was never powerful and only got weaker over time, while Gondor grew in power. So Sauron secretly sent the Lord of the Nazgul to create a dummy evil country to border Arnor called "Angmar", earning him the name the "Witch-king of Angmar".
Then things happened fast: in the year 1974 of the Third Age, Arnor, which had been shrinking for centuries, was destroyed as a country and it's last King, Arvedui, was killed; But Arvedui had a son and everyone knew it, it's just that so many died in the climactic fight with Angmar that their weren't really enough people left in Arnor to make a country. (Angmar also got destroyed, although the Witch-king escaped).
Arnor was destroyed, but the Heirs of Isildur lived on as the wandering Dunedain Rangers. They were (potential) kings with no country.
Gondor got more powerful over the years, but about year 2,000 of the Third Age, there was a King (Earnur) who had no children who decided to meet the challenge of the Lord of the Nazgul in single combat; he died ("never by the hand of man shall he (The Witch King) fall"
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Thus, the Heirs of Anarion, rulers of Gondor for 2,000 years, died out with no survivors.
The Ruling Stewards came to power after this, from a family of prominent nobles who were traditionally aides to the King.
****So you see the Ruling Steward didn't rule in the place of the loss of Isildur, they ruled in the place of the loss of the Heirs of Anarion.
So for 1,000 years before the events of LOTR, Gondor was a kingdom without a king.
*********But Isildur (of Arnor) and Anarion (of Gondor) were brothers, and their father Elendil had been the king of both countries. Because Anarion's line had ended, only a descendant of Isildur could also be held to be the only heir of Elendil---> In the books, Aragorn is called "The Heir of ELENDIL" as much as he is called the "Heir of Isildur".
So at the end of ROTK, Aragorn is not crowned "King of Gondor" he is crowned "King of the Reunited Kingdom of Arnor and Gondor".
Using the resources of still-intact Gondor, Aragorn rebuilds Arnor in the north. The Reunited Kingdom is a vast empire ruled by Aragorn, that included all lands that once belonged to Arnor and Gondor at the greatest extent, excluding only Rohan to which it was permanently allied (Rohan used to be part of Gondor called Calenardhon before it was given away as a gift to the Horsemen of the Wild, the Eotheod, who were then called the Rohirrim and named their new country Rohan; Gondor wanted to creat a new buffer country in a sparsely populated region; Rohan only got created 500 years before LOTR begins).