The Kin-strife (civil war) - Trouble came to Gondor when Valacar, the 20th ruler, married a woman of the Northmen (not of Númenórean descent).

 

"He grew to love the Northern lands and people, and he married Vidumavi, daughter of Vidugavia.  From this marriage came later the war of the Kin-strife.

 

"For the high men of Gondor already looked askance at the Northmen among them; and it was a thing unheard of before that the heir to the crown, or any son of the King, should wed one of lesser or alien race.  There was already rebellion in the southern provinces when King Valacar grew old.  His queen had been a fair and noble lady, but short-lived according to the fate of lesser Men, and the Dúnedain feared that her descendants would prove the same and fall from the majesty of the Kings of Men.  Also they were unwilling to accept as lord her son, who though he is now called Eldacar, had been born in an alien country and was named in his youth Vinitharya, a name of his mother's people.

 

"Therefore when Eldacar succeeded his father there was war in Gondor.  But Eldacar did not prove easy to thrust from his heritage.  To the lineage of Gondor he added the fearless spirit of the Northmen.  He was handsome and valiant, and showed no signs of aging more swiftly than his father.  When the confederates led by the descendants of the kings rose against him, he opposed them to the end of his strength.  At last he was besieged in Osgiliath, and held it long, until hunger and greater forces of the rebels drove him out, leaving the city in flames.  In that siege and burning the Tower of the Dome of Osgiliath was destroyed, and the palantir was lost in the waters.

 

"But Eldacar eluded his enemies, and came to the North, to his kinsfolk in Rhovanion.  Many gathered to him there, both of the Northmen in the service of Gondor, and of the Dúnedain of the northern parts of the realm.  For many of the latter had learned to esteem him, and many more came to hate his usurper.  This was Castamir, grandson of Calimehtar, younger brother of Rómendacil II.  He was not only one of those nearest by blood to the crown, but he had the greatest following of all the rebels; for he was the Captain of Ships, and was supported by the people of the coasts and of the great havens of Pelargir and Umbar.

 

"Castamir had not long sat upon the throne before he proved himself haughty and ungenerous.  He was a cruel man, as he had first shown in the taking of Osgiliath.  He caused Ornendil son of Eldacar, who was captured, to be put to death; and the slaughter and destruction done in the city at his bidding far exceeded the needs of war.  This was remembered in Minas Arnor and in Ithilien; and there love for Castamir was further lessened when it became seen that he cared little for the land, and thought only of the fleets, and purposed to remove the king's seat at Pelargir.

 

"Thus he had been king only ten years, when Eldacar, seeing his time, came with a great army out of the north, and folk flocked to him from Calenardhon and Anórien and Ithilien.  There was a great battle in Lebennin at the Crossings of Erui, in which much of the best blood of Gondor was shed.  Eldacar himself slew Castamir in combat, and so was avenged for Ornendil..." ~The Return of the King (Appendix A)