US Administration bars Israel from attacking Iran,

as announced after Ehud Barak’s meeting with American officials. If true, that may be a significant reason behind Olmert’s resignation.
The US Administration agreed to supply Israel with additional anti-ballistic missile defense systems. Given the time lag before their actual supply, the ABM are not intended to defend Israel against Iran’s reprisal. By the time the ABM are deployed in Israel, it would be already too late to attack Iran.