Gondor reaches the height of its power
Event Type: Political
Age: 3rd Age - The Kings
Years: 1050 ~ 1226
Gondor reached its greatest power under Ciryaher son of Ciryandil, who took the name of Hyarmendacil 'South-victor'.
"In his day Gondor reached the summit of its power. The realm then extended north to [the Field of ] 1 Celebrant and the southern eaves of Mirkwood; west to the Greyflood; east to the inland Sea of Rhûn; south to the River Harnen, and thence along the coast to the peninsula and haven of Umbar. The Men of the Vales of Anduin acknowledged its authority; and the kings of the Harad did homage to Gondor, and their sons lived as hostages in the court of its King."
RotK, Appendix A, I, iv, Gondor and the Heirs of Anarion
1. Note that Christopher Tolkien mentions both in Unfinished Tales and in Peoples of Middle-earth that "to the Celebrant" was an error which should read "to the Field of Celebrant."
According to 'The Atlas of Middle Earth,' "The area under direct rule was probably about 716,425 square miles, with Harad possibly adding another 486,775." The area of the Vales of Anduin is not included in this, as it was not directly governed by Gondor.
A modern equivalent would give a territory under direct rule that was approximately three times the size of the Ukraine; adding Harad would give an area about equal to the size of India.
The Atlas of Middle Earth by Karen Wynn Fonsted, The Third Age, 'Kingdoms of the Dúnedain'
This also marked a time of great vigilance by Gondor, where the fortresses watching Mordor were manned. With the reign of the next king, Atanatar, this vigilance began to lapse.