He made several journeys to Middle-earth and established contact with Gil-galad in his realm in the Grey Havens making friendship and alliance with him. He also established a vital Númenorean port in Middle-earth, named Vinyalonde. He built the great haven of Lond Daer on the river Gwathló, and under his reign the deforesting of Enedwaith and Minhiriath began.
The Realms in Exile was the collective name for the kingdoms, Arnor and Gondor, established by the Dunedain in Middle-earth after they escaped the Downfall of Númenor. For much of the Third Age, they were the dominant powers in the north-west of Middle-earth, and had an enduring political, economic and cultural significance. Initially politically united, they soon became separate kingdoms, but retained much of their common culture, and later entered into an alliance. Arnor broke apart into petty kingdoms, the last of which fell in TA 1974. Gondor survived and, though becoming ever weaker, it remained the most powerful state among the so-called "Free Peoples". After the War of the Ring, Gondor and the former territories of Arnor were merged into the Reunited Kingdom.
Both Arnor and Gondor were feudal absolute monarchies. Their first High King was Elendil the Tall, the leader of the fugitives who escaped during the Akallabêth and a member of the old Númenórean nobility. He ruled Arnor directly, while Gondor was ruled jointly by his sons, Isildur and Anárion, who acted as his vassals. As a result of the War of the Last Alliance (see below), the two kingdoms were politically separated; Arnor was ruled by Isildur's descendants, and Gondor by those of Anárion. In a departure from Númenórean practice, equal primogeniture was abolished in favour of an exclusively male succession, perhaps because of the role the King had in both realms as supreme army commander and the frequent wars in which the realms were embroiled.
While Arnor and Gondor are called "the realms of the Dúnedain", in fact the Dúnedain made up only a small part of their population. The bulk of the human population in the north-west of Middle-earth consisted of native Men and descendants of earlier Númenórean colonists, the two groups having intermingled to varying degrees. The Dúnedain who arrived with Elendil and his sons, being of pure Númenórean blood and culture, were more gifted and powerful than the local inhabitants. Furthermore, the House of Elendil commanded particular respect and loyalty on account of his descent from Elros and his ancestors' leadership of the Faithful in Númenor itself. The Dúnedain therefore quickly established themselves as a sort of hereditary aristocracy, and provided many of the realms' leading statesmen, administrators and military commanders. However, local tribes and lordships, many held by families not of Númenórean descent, persisted, especially away from the great cities. Unlike the earlier colonists, the Dúnedain came to discourage intermarriage with those they termed "lesser Men," fearing perhaps that their gifts would be bred out; this policy eventually had disastrous consequences for Gondor.
Among themselves, the Dúnedain spoke a dialect of Sindarin, in token of their ancient friendship with the Elves, and to set themselves apart from the hostile "Black Númenóreans" who had established lordships in the more southerly regions of Middle-earth. But in dealings with their subject peoples and the surrounding nations, they used Westron, a descendant of the Adûnaic language of Númenor, closely related to many of the Mannish dialects that already prevailed in their territories. In this way, Westron (which also became known as the Common Speech) became the everyday language of most of the inhabitants of the realms, including later immigrants such as the Shire-folk; and even other nearby independent kingdoms and peoples, who had languages of their own, chose Westron for their dealings with foreigners, so that it became the language of trade and diplomacy throughout the West. The Westron was arguably the most enduring cultural legacy of the Realms in Exile.
Arnor and Gondor were founded in SA 3320, the year after the Akallabêth. However, Sauron, who hated Elendil and his family on account of their friendship with the Elves and their opposition to his policies in Númenor, had survived Númenor's downfall, in which he played a leading part. He returned to Middle-earth, and discovered that the power of the Elves under Gil-galad had increased in his absence, and that the Realms in Exile, whose territory extended even to the borders of Mordor, were inclined to side with the Elves in the event of war. Thus, after a mostly peaceful century in which Elendil ruled in the north and Isildur and Anárion in the south, Sauron suddenly attacked Gondor (SA 3429), capturing Minas Ithil and destroying its White Tree. Isildur fled to Arnor, while Anárion defended Osgiliath and Minas Anor. In 3430, Gil-galad and Elendil formed the Last Alliance, initially consisting of the Realms in Exile and the Elf-kingdoms of Lindon and Imladris. The Alliance was soon joined by the Dwarves of Khazad-dûm and the Silvan Elves of Mirkwood and Lórien. The Allies overthrew Sauron's forces at the Battle of Dagorlad, and besieged Barad-dûr for seven years; Anárion, along with many others, was killed during the siege. At last, seeing no other way to break the siege, Sauron himself came forth, and wrestled with Elendil and Gil-galad on the slopes of Mount Doom; they defeated him, but both perished. Isildur cut Sauron's ring of power from his hand and took it for his own. Sauron passed away into the shadows and the Second Age ended.
With both Elendil and Anárion dead, the organisation of the Realms in Exile became murky. Isildur became sole King of both Gondor and Arnor, but instead of seeking to rule both Kingdoms personally, he instructed his nephew Meneldil, the oldest surviving son of Anárion, before making him responsible for ruling Gondor. Isildur, along with his three eldest sons (Elendur, Aratan and Ciryon), then journeyed north, Isildur intending to take up the high kingship of his father in Arnor. Some two thousand years later, Arvedui (Isildur's remote descendant in the north) and the Dúnedain of Gondor disagreed as to whether Isildur relinquished his rights in Gondor, or whether he merely made Meneldil responsible for ruling Gondor under his suzerainty.
Whatever Isildur's plans, they were put to a sudden end when he and his three eldest sons were killed in the Disaster of the Gladden Fields. In that unexpected attack by Orcs, most of Isildur's followers were killed; he himself escaped, taking advantage of the Ring's power of invisibility, but the Orcs tracked him to the Anduin. When he tried to swim across the river, the Ring slipped from his finger, and the Orcs saw him and killed him with arrows. Of his entire army and entourage as he journeyed north, only three eventually reached Arnor; among these was his esquire, who had kept the shards of Elendil's famous sword Narsil. That sword then became an heirloom of the Kingdom of Arnor.
The sceptre of Arnor now came to Valandil, the youngest of Isildur's four sons, who had remained in Rivendell during the war, being still a child when the Last Alliance attacked Sauron; but not until TA 10 did he actually assume his responsibilities. By this time, whatever Isildur's original intentions were, Meneldil in Gondor was a thoroughly independent monarch, and Valandil and his successors in Arnor, badly weakened by Arnor's losses in the war, were in no position to exert authority over Gondor, even if they had wished to do so. So it was that, in place of the unified entity of Elendil's day, two independent kingdoms arose.
It quickly became apparent that Arnor, despite its notional superiority, was by now the weaker of the two realms. Although its territory encompassed almost all of vast Eriador, its population was severely diminished and it never really recovered. In TA 861, Arnor was divided into the three petty kingdoms of Arthedain, Rhudaur and Cardolan, which frequently fought amongst themselves, weakening the northern Dúnedain still further. About TA 1300, an evil power known as the Witch-king established the kingdom of Angmar in the mountainous wastes north of Arthedain and Rhudaur, and proceeded to conquer the realms of the northern Dúnedain one by one. In Rhudaur, power was seized from the Dúnedain in the 14th century by the native Hillmen, who allied themselves with Angmar before eventually being annexed early in the 15th century. Cardolan was fatally weakened by a defeat at the hands of the combined forces of Angmar and Rhudaur in TA 1409, and its few surviving Dúnedain, along with most of its other inhabitants, were eventually wiped out in the Great Plague of TA 1636. Arthedain, the most powerful of the northern kingdoms and the only one in which the line of Isildur was preserved, survived the longest, eventually falling to an all-out invasion from Angmar in TA 1974. After the collapse of Arthedain, the surviving Dúnedain became a wandering people, and secretly guarded Eriador's few surviving settled communities, such as the Shire and Bree, against evil and wild creatures.
In the south, Gondor grew and flourished for a thousand years, gaining territory to the north-east (southern Rhovanion), the north-west (Enedwaith) and the south (Umbar). It seems Gondor never sought to expand further northwest than the Greyflood, and no records survive of conflict between Gondor and unified Arnor, or between Gondor and Cardolan. But nor are there any records of alliance or friendly communication; Gondor seems rather to have been preoccupied with the peoples to its east and south, and with the threat of Mordor, which Gondor guarded with a series of fortresses. Thus Gondor came to control a vast territory by the time of Hyarmendacil I, but like Arnor it slowly decayed through a combination of internal weaknesses and external threats. Its Dúnedain population slowly shrank, the southern Dúnedain tending to marry late and have few children, so that Gondor gradually lost its main source of strength compared to the surrounding nations. During the second millennium of the Third Age, Gondor was hit by three great calamities. The first was the Kin-strife, a civil war brought about by the marriage of King Valacar to a foreign woman and the accession of their son; during the Kin-strife, the Dúnedain aristocracy was decimated, the great palantír of Osgiliath lost, and Umbar was wrested from Gondor by the survivors of the rebel faction. The second, the Great Plague that wreaked havoc in Cardolan (see above), had earlier killed great numbers of the people of Gondor. The third calamity, a century-long war with the Wainriders, cost Gondor its eastern territories and nearly exterminated the royal family.
In light of the threats of Angmar and the Wainriders, Gondor and Arthedain made an alliance in TA 1940, the first such alliance for many centuries, and cemented it by the marriage of Arvedui, heir to the throne of Arthedain, and Fíriel, daughter of Ondoher of Gondor. When Ondoher and his two sons were killed in battle with the Wainriders four years later, Arvedui claimed the now vacant throne of Gondor, by virtue of his descent from Isildur and secondarily in right of his wife. Had his claim been accepted, it seems likely that a Reunited Kingdom would have emerged far earlier, and to the advantage of both North and South. But the Council of Gondor, perhaps offended by his assertion of the rights of Isildur's descendants in Gondor, and certainly seeing the northern prince as a weak candidate, instead offered the crown to Eärnil, the general responsible for victory over the Wainriders and a distant cousin of Gondor's late King. Despite the obvious snub the Council thus offered to the northern Dúnedain, Eärnil II maintained Gondor's alliance with Arthedain, going so far as to send an expeditionary force north to help Arvedui (who had by then succeeded his father) against Angmar; but that fleet arrived too late to save Arthedain, instead providing the bulk of the forces that would smash Angmar at Fornost. In that encounter, Eärnur, Eärnil's son and eventual successor, was personally humiliated by the Witch-king. When therefore the Nazgûl had conquered Minas Ithil (TA 2002), and Eärnur succeeded his father, the Witch-king, appealing to his pride, challenged him to single combat; he rode to the gates of Minas Morgul (as Minas Ithil had been renamed) with a small retinue and was never seen or heard from again.
Thus, within two generations of the Dúnedain, both Arnor and Gondor lost their kings. The northern Dúnedain were too weak to maintain a kingdom, and so their leaders, the successors of Isildur, took the humble title, "Chieftain of the Dúnedain". In the south, by contrast, Gondor was still a viable state, but the male-line descendants of Anárion were all but extinct. Unlike Rhudaur and Cardolan in the north, the Dúnedain of Gondor refused to look for a king outside the royal family, so power was exercised in the name of the king by 26 successive Ruling Stewards, the first of whom was Mardil the Steadfast. By the time of the later Stewards, Sauron's forces were constantly encroaching on Gondor's borders, and several more territories were ceded to allies or conquered by enemies: Calenardhon in the 26th century, and Ithilien, Gondor's sole remaining province east of the Anduin, in the 30th. In the War of the Ring, Sauron sought to smash Gondor, his strongest opponent, but forces from Gondor and other nations beat his armies back in the Battle of the Pelennor Fields. During that battle, Aragorn II, the Chieftain of the Dúnedain of the North, and thus a descendant of both Isildur and Anárion, distinguished himself by arriving with a strong relief army at the critical moment, and was acclaimed by the people of Gondor as King; his only notable opponent within Gondor's ruling classes, Ruling Steward Denethor II, was driven mad and committed suicide on the day of the battle. Thus, after Sauron was defeated by the destruction of his Ring, Aragorn was crowned King of Gondor and received the sceptre of the Kings of Arnor, becoming the first King of the Reunited Kingdom.