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The actual war began with the assault upon Minas Ithil in SA 3429. When the city was lost, Isildur and his family escaped to Osgiliath. From there they sailed away, leaving Anarion to defend the realm. It may be that at this time Isildur stopped at Edhellond, and passed northward to Erech to call upon the King of the Mountains to fulfill the oath his predecessors had sworn; or, it may be at this time Isildur extracted the oath from the King, to be fulfilled later when the West was ready to march against Sauron.
In any event, Anarion apparently was not troubled by the men of Ered Nimrais, though he may have prudently mounted a watch against betrayal from the west. The presence of the Elven haven at Edhellond may have been a comfort to the Dunedain, however. Also, since Sauron had gathered his allies in Mordor, the armies of Herumor and Fuinur did not attack from the south. They must have passed north into Mordor through the Nargil Pass, or else they marched northward along the Ephel Duath to assist in the assault on Osgiliath.
Elendil and Gil-galad held a council in SA 3430 where the Alliance was formally created. The council must have been quite a large gathering of lords from Arnor, Gondor, Lindon, and other lands. Besides Gil-galad and Elendil we can guess that Isildur and Cirdan were in attendance, and perhaps also Elrond, Celeborn, Galadriel, Gildor Inglorion, and Glorfindel. Isildur’s sons Elendur, Aratan, and Ciryon might also have been in attendance; at the very least Elendur was probably there.
There may also have been emissaries from the Vales of Anduin (if not Durin IV, Oropher, and Amdir themselves). Possible emissaries from the Elves would include Thranduil and Amroth. However, it may be that the Alliance originally included only Lindon, Arnor, Gondor, and Imladris.
The Alliance could only have had one military purpose: march upon Mordor and achieve a complete and total victory against Sauron. They knew they could beat him in the field, as this had been accomplished on more than one occasion in past wars. The real question must have concerned what they would do once they breached Sauron’s defenses. How long could he hold out against the Alliance, and what would he be able to contrive against his enemies while besieged in Barad-dur? Sauron’s forces were considerable, for he commanded not only the Orcs and Trolls, but also many Men, and his chief servants were the Nazgul.
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