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[Mat_Heathertoes]

Some of the Valar were indeed wise to that peaching sneakthief Melkor .. Ulmo for one and Tulkas was another

But Ulmo was not deceived, and Tulkas clenched his hands whenever he saw Melkor his foe go by .....

Silmarillion - Chapter 6 - Of Fëanor

But they had to abide by the will of Manwë

For Manwë was free from evil and could not comprehend it, and he knew that in the beginning, in the thought of Illúvatar, Melkor had been even as he; and he saw not to the depths of Melkor's heart

Silmarillion - Chapter 6 - Of Fëanor

So a sense of brotherly love and the goody-goodyness of Manwë was the undoing of the Valar at that time. But as it is mentioned elsewhere many times everything that did happen was doomed to happen in any case and Mandos knew it. But the war between good and evil was the fate of Arda in its conception and even Eru knew that...

'...And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.'

Illúvatar to Melkor - Ainulindalë