This is the in-depth story background at the start of the first game.
By the time of LOTR, there is only one major human Realm left for the good guys- Gondor. The Rohirrim inhabit Rohan, but too sparsely for it to be a true Realm. Humans live up in Eriador in towns like Bree but nowhere strong enough to be called a truly powerful force.
But it was not always so. When the fabled Dunedain Kingdom of Numenor (a large island to the west of the places seen in LOTR) was destroyed in the waning years of the Second Age, TWO human realms were formed in Middle-Earth by the prime survivor, Elendil. Gondor in the south, and Arnor in the north.
Elendil is one of the greatest human heroes of all time, wielder of the sword Narsil. He formed The Last Alliance with the Elves ("last" not because they would never again be friends, but because there would never again be enough Elves in Middle-Earth to form a true alliance), along with the great Elven ruler Gil-Galad. These two mighty and nigh-on unstoppable heroes fought back the forces of Sauron until that battle seen at the start of Fellowship of the Ring. There, both heroes were slain by Sauron himself. But Isildur, Elendil's son, took his father's sword and cut the Ring from Sauron's hand (and meanwhile Gil-Galad's talented standard bearer, Elrond, took Gil-Galad's ring and became a great Elevn ruler in his place).
Isildur, at the start of the Third Age, was now King of Arnor, with a secure line and good prospects for both Kingdoms. But in a disastrous ambush brought on by ill-caution and his belief that evil was vanquished, the Ring betrayed him. He and his three eldest sons died. In the confusion and chaos of the next ten years, seperate Kings took the thrones on both Kingdoms- both descendants of Elendil, but now parted from each other. Despite the Palantirs that could keep communications going, Arnor and Gondor lose contact, and each may as well be on the other side of the world from the other.
Whilst Gondor grew more powerful, Arnor was ever beset by troubles. Eight centuries later a disastrous argument over succession meant that King Earendur was succeeded by all three of his sons. Arnor was split into three smaller Kingdoms- Rhudaur, to the north-east, Cardolan, to the south, and Arthedain, to the north-west. In nearby Rivendell, Elrond watched sorrowfully as the descendants of his brother squabbled. Relations between Rivendell and Arnor grew icy. The three Kingdoms tried to maintain close ties but it was always hard- there was always the suspicion that one would try to re-unify by force.
Two centuries later, Sauron is re-born, and while he will not make his grand move for another two thousand years, he lays plans to tear down these human Kingdoms far in advance of the War of the Ring. His prime objective is the total destruction of Elendil's line, which involves wiping out all the Kingdoms of man and their royal families.
No-one knows of this coming Shadow except five Wizards who arrive in Middle-Earth at this time. The ones men name Saruman and Gandalf make it their business to acquaint themselves with both Kingdoms, and try to advise them. Arnor is often too split to listen.
The true evil starts some three hundred years after that with the foundation of the mountain kingdom Angmar to the north-east of Arnor. Its evil leader, the Witch-King (in fact the Chief Nazgul, though no-one knows this) is preparing to put into operation his master's plan to destroy Arnor and wipe out the royal families there. Sauron's eye will not be open again for some time, but the Witch-King is a cunning, powerful and dangerous opponent who is more than ready to do all the thinking himself.
Arnor soons learns of the threat to its borders. Watchful armies prepare for the Witch-King's invasion, just as Gondor keeps watch on Mordor. The Witch-King gathers a fearful army of orcs, and a human army of Dunlendings (the hill-people seen briefly in TTT, who have always hated the Dunadain and their kingdoms). And the Witch-King has many spies and informers, and dread powers.
At the worst of times, the three kingdoms still squabble amongst themselves. The Witch-King's efforts keep them divided. Rhudaur falls apart internally, its royal family virtually usurped by an evil Dunlending Lord who whips this weakest of the tree Kingdoms into war with its neighbours. The King of Arthedain has already been killed in battle with Rhudaur before the Dunlending's alliance with the Witch-King (still a mysterious figure living only in rumour) is revealed and it is already too late.
A vast Dunlending army overwhelms the remaining loyal defenders of Rhudaur. The orcs move subtley, at night, striking fast and hidden. Rhudaur is destroyed, their capital taken by the Dunlendings, the last true king of Rhudaur killed just 55 years after the Witch-King began his plans.
Next it is Cardolan's turn. Here, too, large armies tear into its rich fields. A vast battle will be fought to decide things, at the site of the centre-point of the three Kingdoms- the tower of Amon-Sul on Weathertop (the place where the Nazguls waylay Frodo and Aragorn in FOTR). The remnants of the armies of Rhudaur, and the armies of Cardolan and Arthedain are drawn up against the Witch-King's forces, in the year 1409.
But it does not quite go as hoped. Arthedain decides the battle is a lost cause. Its army turns up before everyone else's, takes the sacred Palantir from the tower, and then buggers off back home, abandoning the others to their fate. They are defeated. Arthedain can claim to have saved the Palantir, but Cardolan will never forgive the treachery, though Arthedain still fights the Witch-King, losing another King in battle to his forces near this time.
Cardolan is shattered by the invasion, and its own line of Kings destroyed three years later. Yet it holds. The Witch-King's armies never manage to occupy it, always being driven out by rebellions, intervention from Arthedain, and, at last, the Elves. Elrond sends an army to fight the forces of Angmar and drives them back to their homeland- though Dunlendings still hold Rhudaur. With the Elves keeping watch on him, the Witch King settles into a long wait.
Cardolan never truly re-builds. Its spirit is lost as various minor Lords vie to form a new line of Kings, only unifying in their determination not to let Arthedain rule them! Over two centuries it grows weaker and weaker still until it is barely a country at all, more a loose association of people. And at this time the great plague, which also troubled Gondor greatly, struck. Among its casualties are the entire Cardolanian royal family. Two down for the Witch-King- one to go.
Gondor too has its many troubles at this time- plague, war, politics- and neither Kingdom is ever strong enough to help the other. Barely any news travels between the two at all.
(And about this time, the Hobbits arrive in Arthedain, asking for a land of their own, which the king grants them, so long as they maintain the roads and bridges there, which they do. The King of Arthedain becomes their King. This land will later become the Shire.)
Arthedain does well in these centuries. In fact, many simply refer to it now as Arnor, for it is now simply a smaller version of the glory that was, even though the term literally means all three kingdoms, one fallen, one shattered, and one whole. The Witch-King tries but fails many times to break the defences. Arthedain has become greatly experienced in fighting Dunlendings and orcs, in rooting out spies and traitors, in defeating evil in all its forms. But on the other hand, Angmar also remains untouched.
The plague is now gone from Cardolan, but is is feebly weak. As a slow darkness grows through Middle-Earth, the Elves are overstretched and the Witch King's power is growing. Their watch on Angmar ceases and the Witch-King prepares to move again. Many feel the Witch-King will now finally occupy Cardolan forever. If his armies now become placed on TWO borders of Arthedain, it could be too much to bear.
But Arthedain remains solid, and its King, Arvedui, is talented, intelligent, and strong. Arevdui has re-opened relations with Gondor for the first time in centuries, Arvedui cementing a new era of friendship by taking a Gondorian Princess as his wife. When Gondor recently had a succession crisis with its King Arevdeui had a good claim for the crown himself, being both descended from Elendil and now connected to Gondor my marriage. Arvedui contacted Gondor and offered to take the throne himself. This may have been a good idea, but the proud Gondorians refused him. Nonetheless, with the long silence between the two Kingdoms broken, maybe with a strong alliance between Arnor and Gondor the evil can be driven from Middle-Earth.
These are the times you live in. The dreaded Witch-King prepares to move again, to occupy Cardolan and then tear down Arthedain. Arthedain prepares to respond but is unsure how. Maybe Elrond can be persuaded to help? Maybe the alliance with Gondor will bear fruit? In the valley of the river Anduin to the east, the Eotheod (later the Riders of Rohan) have overthrown an occupation by the Easterlings and grow strong again- the Horse Lords hate the Orcs and Dunlendings and are fierce warriors all, may they even help? Are they strong enough to make any difference? And the other peoples of the North, the Lossoth and Beornings? Will they just stand by?
The future of Arnor is in the balance. If it falls the whole of the north-west of Middle Earth will fall to evil, except maybe Rivendell. Mirkwood- once the Greenwood, but re-named since the evil of the Witch-King infected it- is already overcome of foul beings, and if the Witch-King is victorious, Gondor will be soon faced by a vast enemy to east AND north, and will never survive Sauron's resurgence.
It is in these days that your heroes journey to Arnor, and will have much to say on what happens next.