Tehillim

The Rabbonim in this great city have called for a public meeting for the recitation of Tehillim. Good for them.
Why did they wait till now? Were missiles falling on Sderot over the past years not enough of a reason to call for this event earlier?
What do Tehillim do? Are they intended to influence Heaven into changing the direction of the war? I was under the impression that Heaven cannot be influenced, at least that's what many philosophers stated until they realized that huge parts of our tradition are dependent on the potential for Heaven to be influenced.
Is saying Tehillim intended to mollify the guilt one feels at not being really able to do anything that will influence the situation on way or another?
Do we say Tehillim again just in case they will make a difference this time?
Why did the Tehillim which surely have been recited by these Rabbonim not prevent this calamity from occurring in the first place?