§ Of the Fifth Battle: Nirnaeth Arnoediad The Silmarillion

From Doriath came little help. For Maedhros and his brothers, being constrained by their oath, had before sent to Thingol and reminded him with haughty words of their claim, summoning him to yield the Silmaril, or become their enemy. Melian counselled him to surrender it; but the words of the sons of Fëanor were proud and threatening, and Thingol was filled with anger, thinking of the anguish of Lúthien and the blood of Beren whereby the jewel had been won, despite the malice of Celegorm and Curufin. And every day that he looked upon the Silmaril the more he desired to keep it for ever; for such was its power. Therefore he sent back the messengers with scornful words. Maedhros made no answer, for he had now begun to devise the league and union of the Elves; but Celegorm and Curufin vowed openly to slay Thingol and destroy his people, if they came victorious from war, and the jewel were not surrendered of free will. Then Thingol fortified the marches of his realm, and went not to war, nor any out of Doriath save Mablung and Beleg, who were unwilling to have no part in these great deeds. To them Thingol gave leave to go, so long as they served not the sons of Fëanor; and they joined themselves to the host of Fingon.