Were the Valar Shirking their Duty?

[Valandil]

Here's what always strikes me about "The Silmarillion": It seems like the Valar were sent to Arda to prevent Melkor from ruining it (in addition to fulfilling the creation of it). Instead, it seems like they despaired at his destruction - and set up their own little section where they would make sure and run things right - and the rest could just go to pot.

When the Elves came along, the response of the Valar was to bring them over to this sheltered land they had created - instead of preventing Melkor from corrupting the land where they were placed (and, perhaps the Elves themselves).

I never quite understood this. Melkor was greater than any single Vala... but if they took him on together, he didn't stand a chance - as they proved at the end of the First Age. Why didn't they just defeat him from the start? Or at least 'take him out' once the Firstborn came along?

Of course... then we wouldn't have a story. Were the Valar doing the 'right' thing though - what Eru would have wanted them to do? Or were they proving to be as 'fallable' in their own way as Elves and Men by not always doing the correct thing? Didn't they always have responsibility for all of Arda? And did they fail in this responsibility by allowing Melkor to go unchecked?

(In fact, along these lines, it could be argued that the rebellion of the Elves against the Valar ultimately induced them to do what they should have done to begin with.)