


After the Fall of Numenor, the faction of the faithful led by Elendil and his sons fled via ships to Middle Earth from the destruction. However, the rough seas separated Elendil from his sons.
Elendil arrived in the north where he founded the Kingdom of Arnor, and his sons Isildur and Anarion arrived in the south founding the Kingdom of Gondor.
After establishing contact between one another. Elendil’s sons submit to their father as “High King” of both kingdoms but he remains in Arnor while they co-rule Gondor as brothers. Then Sauron attacks Gondor (he hated the Numenoreans) and starts the War of the Last Alliance.
After the War of the Last Alliance (the opening scene of the Fellowship of the Ring) Elendil and his younger son are both killed leaving Isildur the sole heir to the title of High King of Arnor and Gondor, but on his way back to claim his fathers throne in the north this happens:
Isildur’s son Valendil was just a child at the time of his fathers death on the road and it took years for his handlers (Elrond) to help him stabilize the north.
Meanwhile his cousin Meneldil (the son of Isildur’s brother Anarion) was crowned king of Gondor in the south.
Meanwhile his cousin Meneldil (the son of Isildur’s brother Anarion) was crowned king of Gondor in the south.
Therefore, Valendil never pursued his claim on Gondor and both kingdoms decided to continue on as separate entities but close allies. Arnor in the north being ruled by the descendants of Isildur and Gondor being ruled by the descendants of Anarion in the south.
As the third age progressed with this political dynamic. The Witch King re-emerged in Angmar (under the secret orders of Sauron) waged war on the northern kingdom of Arnor.
The war was long and brutal. It spanned hundreds of years and culminated with the Witch King crushing the last king of Arnor, Arvedui. The northern kingdom was destroyed and Arvedui ended up drowned in a shipwreck trying to escape out of the north.
Arvedui’s young son Aranarth became the first chieftain of the Dunedain rangers as there was no kingdom left to rule; a title which would eventually fall to Aragorn. And that is where we find the House of Isildur in the Fellowship of the Ring.
After destroying the northern kingdom the Witch King is himself defeated by a coalition army of Gondor and Elrond, but he left the north and returned to Mordor where he set up shop in Minas Morgul (the glowing green city from the movies)
While at Minas Morgul he challenged the new young King Eänur of Gondor to a duel. Eänur was foolish and accepted the challenge and road to the city of sorcery with a small company of knights and was never seen again.
He was slain by the With King with no heir. Therefore rule of Gondor was left to the Stewards, care takers of the throne until an heir to Elendil would return. And that’s where we are left at the Fellowship of the Ring
I love this political explanation for Aragorns claim to the throne. The line of Aragorn and how it all unfolded adds an interesting, somewhat historically realistic display of dynastic succession between allied kingdoms.