[Maédhros]

Good point, Phantom. The Valar made a very serious and rather stupid mistake in letting Morgoth go free once he was caught the first time. Nienna's pity makes her a bit too trusting.

I have to totally disagree with this.
From Ósanwe-kenta

Who among the Eldar hold that the captivity of Melkor in Mandos (which was achieved by force) was either unwise or unlawful? Yet the resolve to assault Melkor, not merely to withstand him, to meet violence with wrath to the peril of Arda, was taken by Manwë only with reluctance. And consider: what good in this case did even the lawful use of force accomplish? It removed him for a while and relieved Middle-earth from the pressure of his malice, but it did not uproot his evil, for it could not do so. Unless, maybe, Melkor had indeed repented. But he did not repent, and in humiliation he became more obdurate: more subtle in his deceits, more cunning in his lies, crueller and more dastardly in his revenge. The weakest and most imprudent of all the actions of Manwë, as it seems to many, was the release of Melkor from captivity. From this came the greatest loss and harm: the death of the Trees, and the exile and the anguish of the Noldor. Yet through this suffering there came also, as maybe in no other way could it have come, the victory of the Elder Days: the downfall of Angband and the last overthrow of Melkor.
Who then can say with assurance that if Melkor had been held in bond less evil would have followed? Even in his diminishment the power of Melkor is beyond our calculation. Yet some ruinous outburst of his despair is not the worst that might have befallen. The release was according to the promise of Manwë. If Manwë had broken this promise for his own purposes, even though still intending "good", he would have taken a step upon the paths of Melkor. That is a perilous step. In that hour and act he would have ceased to be the vice-gerent of the One, becoming but a king who takes advantage over a rival whom he has conquered by force. Would we then have the sorrows that indeed befell; or would we have the Elder King lose his honour, and so pass, maybe, to a world rent between two proud lords striving for the throne? Of this we may be sure, we children of small strength: any one of the Valar might have taken the paths of Melkor and become like him: one was enough.