The Silmarillion

But Turgon remembered the city set upon a hill, Tirion the fair with its tower and tree, and he found not what he sought, but returned to Nevrast, and sat in peace in Vinyamar by the shores of the sea. And in the next year Ulmo himself appeared to him, and bade him go forth again alone into the Vale of Sirion; and Turgon went forth, and by the guidance of Ulmo he discovered the hidden vale of Tumladen in the Encircling Mountains, in the midst of which there was a hill of stone. Of this he spoke to none as yet, but returned once more to Nevrast, and there began in his secret counsels to devise the plan of a city after the manner of Tirion upon Túna, for which his heart yearned in exile.

Now Morgoth, believing the report of his spies that the lords of the Noldor were wandering abroad with little thought of war, made trial of the strength and watchfulness of his enemies. Once more, with little warning, his might was stirred, and suddenly there were earthquakes in the north, and fire came from fissures in the earth, and the Iron Mountains vomited flame; and Orcs poured forth across the plain of Ard-galen. Thence they thrust down the Pass of Sirion in the west, and in the east they burst through the land of Maglor, in the gap between the hills of Maedhros and the outliers of the Blue Mountains. But Fingolfin and Maedhros were not sleeping, and while others sought out the scattered bands of Orcs that strayed in Beleriand did great evil they came upon the main host from either side as it was assaulting Dorthonion; and they defeated the servants of Morgoth, and pursuing them across Ard-galen destroyed them utterly, to the least and last, within sight of Angband's gates. That was the third great battle of the Wars of Beleriand, and it was named Dagor Aglareb, the Glorious Battle.

A victory it was, and yet a warning; and the princes took heed of it, and thereafter drew closer their leaguer, and strengthened and ordered their watch, setting the Siege of Angband. which lasted wellnigh four hundred years of the Sun. For a long time after Dagor Aglareb no servant of Morgoth would venture from his gates, for they feared the lords of the Noldor; and Fingolfin boasted that save by treason among themselves Morgoth could never again burst from the leaguer of the Eldar, nor come upon them at unawares. Yet the Noldor could not capture Angband, nor could they regain the Silmarils; and war never wholly ceased in all that time of the Siege, for Morgoth devised new evils, and ever and anon he would make trial of his enemies. Nor could the stronghold of Morgoth be ever wholly encircled: for the Iron Mountains, from whose great curving wall the towers of Thangorodrim were thrust forward, defended it upon either side, and were impassable to the Noldor, because of their snow and ice. Thus in his rear and to the north Morgoth had no foes, and by that way his spies at times went out, and came by devious routes into Beleriand. And desiring above all to sow fear and disunion among the Eldar, he commanded the Orcs to take alive any of them that they could and bring them bound to Angband; and some he so daunted by the terror of his eyes that they needed no chains more, but walked ever in fear of him, doing his will wherever they might be. Thus Morgoth learned much of all that had befallen since the rebellion of Fëanor, and he rejoiced, seeing therein the seed of many dissensions among his foes.

When nearly one hundred years had run since the Dagor Aglareb, Morgoth endeavoured to take Fingolfin at unawares (for he knew of the vigilance of Maedhros); and he sent forth an army into the white north, and they turned west and again south and came down the coasts to the Firth of Drengist, by the route that Fingolfin followed from the Grinding Ice. Thus they would enter into the realm of Hithlum from the west; but they were espied in time, and Fingon fell upon them among the hills at the head of the Firth, and most of the Orcs were driven into the sea. This was not reckoned among the great battles, for the Orcs were not in great number, and only a part of the people of Hithlum fought there. But thereafter there was peace for many years, and no open assault from Angband, for Morgoth perceived now that the Orcs unaided were no match for the Noldor; and he sought in his heart for new counsel.