Originally I believe the plan was to attack Iran first - that was in the papers they had mapped out back then. At some point they must have decided that Syria (along with other regimes in the Middle East that they didn't trust like Libya) had to fall first. But I think they didn't expect the Assad regime to last more than a decade after a color revolution... The events leading up to Assad's fall last year do suggest that they were sick of waiting and wanted to fast track the process. I also think the 2008 crash and related events might have delayed things for a while too, because the Arab Spring wasn't put into motion until 2011.