At its greatest, Arnor encompassed almost the whole region of Eriador between Bruinen, Gwathló and Lhûn, and the region which would later be known as the Shire. Arnor's population was composed by Dúnedain in western-central regions and mixed or indigenous (and reluctant as citizens) peoples. Its capital was Annúminas near the lake Nenuial.
Before the foundation of Arnor there was already a sizable Númenórean population living there, a result of the slow emigration of Númenóreans which had started under Tar-Meneldur and especially Tar-Aldarion. Most of them lived in the harbour of Vinyalondë, later called Lond Daer. Before the arrival of the Dúnedain Arnor was home to Middle Men of Edain stock, and the early colonists soon interbred with the indigenous population. Arnor was originally favoured over the more southern regions (Gondor) because the Elves under Gil-galad lived near it across the river Lhûn. However, in later days after the Númenóreans fell under Sauron's shadow they settled more to the south in portcities like Pelargir and Umbar. This led to a situation where Elendil arrived in an area populated by people who, unlike his own Númenóreans, were mainly still friends with the Elves, and unlike Gondor to the south in Arnor much knowledge of the Elder Days was preserved.
Arnor's second king Isildur (also King of Gondor) was killed in by Orcs in the disastrous Battle of the Gladden Fields. His three eldest sons, among them Elendur that could have been the most magnificent ruler of the Dúnedain exiles, were killed with him, but the fourth and youngest, Valandil, having been left in Rivendell being too young to go to war, survived and became king of Arnor ().
Because Valandil and his heirs did not claim the throne of Gondor the realms were split, but Arnor's ruler kept the title High King, whereas in the south the ruler was 'just' King.
With the victory of the War of the Last Alliance, Arnor's power reached its zenith. The King of Arnor held the overlordship of all the land between the Ered Luin and the River Poros on the southern borders of Ithilien, east to the eastern borders of Mordor. After the Disaster of the Gladden Fields, Arnor began its slow decline. In many ways, Arnor never truly recovered from this disastrous blow to its manpower: While the dawn of the Third Age heralded a new era of opportunity and expansion for Gondor, Arnor began it in a futile attempt to simply recover lost strength.
This marked the start of the decline of the Northern Dúnedain. The three kingdoms had frequent border skirmishes over boundary disputes, but the relationship of Arthedain and Cardolan remained relatively peaceful. However, Rhudaur was unfriendly towards the two other successor states, and took part in a bitter conflict with Cardolan over the tower of Amon Sûl and the palantír associated with the tower.
Around , an evil power, the Witch-king, arose in the mountains of the North East, where he founded Angmar. It was later revealed he was the leader of the Nazgûl, who were dispersed after the first overthrow of Sauron in at the hands of the Last Alliance of Elves and Men, but survived nonetheless.
The last Kings of Rhudaur were not of Númenórean blood, but were descended of Men in service of Angmar. Under their rule the land became a vassal of Angmar, and thus enemies of Cardolan and Arthedain (the two remaining kingdoms). Angmar annexed and terminated the kingdom in . By this time the Dúnedain were gone from the region, as well as most of the other inhabitants. Constant battle with Angmar began to wear down the forces of Arthedain, while Cardolan as a political entity came to an end shortly after 1409.
In theory Arthedain became Arnor with the end of Cardolan and Rhudaur, but the Kingdom was a poor reflection of the large and powerful realm of Arnor before 861. Arthedain was destroyed in 1974, culminating in the pyrrhic victory of the Battle of Fornost. A coalition of Elves, the forces of Gondor and the remainder of Arnor's armies destroyed Angmar and freed Fornost. The population of Arnor were mostly wiped out by the wars, but the Hobbits survived in the Shire, men survived in Bree and probably other villages, and the Dúnedain of Arnor created new homes in the Angle south of Rivendell, where some of them became known as the Rangers of the North. With the help of the Elves, those people remained hidden from the rest of Middle-earth, and became an isolated, wandering people.
After it became a kingdom, Arthedain nominally included the Shire. The kingdom's capital was at Fornost, and Bree was one of its important towns. Annúminas was in the territory of Arthedain, but mostly abandoned and falling into ruin.
Around T.A. 1300 the kingdom of Angmar appeared at Arthedain's north-eastern border. Its King was the chief of the Ringwraiths, the Witch-king, although this was not known to the Dúnedain. Rhudaur, aided by Angmar, attacked in T.A. 1356. Argeleb I died in this conflict. When this new threat came Cardolan placed itself under the suzerainty of Arthedain, which then began to call itself Arnor again. Cardolan repeatedly sent aid to Arthedain when needed but by T.A. 1409 Cardolan and Rhudaur were conquered by Angmar and Arthedain only survived with the help of Elvish reinforcements from Lórien and the Havens.
After 1409 Angmar's power was temporarily broken and the North Kingdom enjoyed relative peace although its population continued to decline. Indeed the decline was so severe that in 1601 Argeleb II granted a large portion of Arthedain's best farmland to Hobbit refugees, as these lands had become deserted. Arthedain was not badly affected by the Great Plague and it can be guessed that warfare with Angmar resumed in earnest sometime after 1800, with mention being made of a victory gained in 1851 by King Araval, although sources for the Kingdom's history are sketchy until the beginning of the 20th century of the Third Age. Arthedain spent the next seventy years in desperate conflict with Angmar but the Northern realm probably lacked the manpower and resources to win the war, as aid from Gondor was not forthcoming owing to the Wainrider threat. In T.A. 1974 Arthedain was overrun and Fornost fell into the hands of Angmar. King Arvedui fled northwards and was killed in a ship wreck the following year, taking with him the palantíri of the North. Arthedain fell just one year too soon for help was finally sent from Gondor under the lead of Eärnur. This army reached Arthedain in T.A. 1975 and destroyed Angmar at the Battle of Fornost.
The death of Arvedui spelt the end of the North Kingdom. The kings of Arthedain were also the lords of the Shire and the Shire chose the Thain to replace the kings. His son, Aranarth decided to not rebuild the kingdom and so became the first Chieftain of the Dúnedain. From him Aragorn is descended.
After it became a kingdom, Cardolan also claimed the Arthedain-controlled Weather Hills, which contained the fortress of Amon Sûl (Weathertop) and its valuable palantír. For this reason the Weather Hills were claimed by all three kingdoms — Arthedain, Cardolan and Rhudaur. This territorial dispute continued until Rhudaur became a vassal of Angmar after the line of the Dúnedain kings failed there.
In T.A. 1050, the branch of Hobbits known as the Harfoots crossed the Misty Mountains, and settled in the South Downs in the west of Cardolan. They were joined about a century later by the Fallohides.
When the kingdom of Angmar arose in northern Eriador, Cardolan became the most important ally of Arthedain. It had to fight the combined armies of both Angmar and Rhudaur. In 1356 Argeleb I of Arthedain was slain in battle with Rhudaur, now allied with Angmar. For a while Cardolan and Arthedain held back Angmar, and in its last years the people became entrenched in their capital region, Tyrn Gorthad, but in 1409 a large Army from Angmar broke into Cardolan and devastated the country. Arthedain could provide little aid, as it was itself under attack. The last King of Cardolan died in this conflict, and Cardolan was shattered. While Arthedain recovered something of her power, Cardolan did not and the region of the Barrow-downs entered hobbit legend as a place of mystery and danger.
In 1636 the Great Plague claimed the life of the King of Gondor, and withered the White Tree. The plague spread north along the Great Road that joined the two kingdoms, and the population of Minhiriath was decimated. About this time the plague also wiped out the Dúnedain hiding in the Barrow-downs and evil spirits came to dwell there. What few folk survived could offer little aid to Arthedain in 1974, when Angmar overwhelmed the last of the kingdoms of Arnor. Until the end of the Third Age, the Dúnedain of Cardolan were only a memory, their tombs and barrows haunted by evil wights sent from Angmar; for the Rangers that wandered over the lands were descended from the people of Arthedain. The only major settlements in old Cardolan were likely to be found along the North-South Road close to Tharbad, until 2912, when terrible floods devastated the lowlands and destroyed Tharbad.
The name Cardolan appears to be dialect Sindarin for "Red Hill Land".
The land between the rivers Hoarwell and Bruinen was also considered part of Rhudaur. It was called the Angle, and it is here that the first Stoor Hobbits came into Eriador around T.A. 1150. However, due to the increasing hostility of Angmar these Stoors fled the region around T.A. 1356, with some of them moving west to the Shire, and others moving back to Wilderland.
Rhudaur's Dúnedain population was always small, and was always only a small proportion of its people. From the start of its existence as a separate kingdom, Rhudaur was unfriendly towards the two other successor states, and took part in a bitter conflict with Arthedain over the tower of Amon Sûl and the palantír associated with the tower.
Over time, the more numerous Hillmen came to dominate the Dúnedain of Rhudaur: the latter were either killed or driven off. In T.A. 1356, they slew the High King Argeleb in battle; the Stoors (fearing Angmar) fled south into Dunland, or east over the mountains into the Vale of Anduin. Angmar annexed and terminated the kingdom in T.A. 1409, at which point the last Dúnedain fled the region.
The Great Plague of T.A. 1636 devastated Eriador. This stemmed the tide for 300 years, because Rhudaur and Angmar were not spared. But the most telling blow was struck in the year T.A. 1975. Arthedain and Cardolan had fallen to the combined hosts of Rhudaur and the Witch-king the previous year, but these hosts were themselves wiped out by a combined army of Gondor and Lindon. The Witch-king fled to the North, and the Hillmen vanished from the histories of Middle-earth. As far south as the Great East Road, Rhudaur became a troll-country; travellers along the Road generally hurried along their way and avoided the Trollshaws.
There is evidence that after the fall of Angmar at the Battle of Fornost the Angle became home to the remainder of the Dúnedain, and the Rangers of the North established several villages there, where their people lived until the resurrection of the northern Kingdom under King Elessar at the end of the Third Age. But northern Rhudaur remained wild and dangerous for the rest of that Age: Arador was slain here by hill-trolls in T.A. 2930, and his son Arathorn II fell in battle with Orcs in T.A. 2933. And in T.A. 2941, trolls captured the company of Thorin at the start of The Hobbit.