By **ah Browning and Angus McDowall DUBAI/RIYADH, June 24 (Reuters) - Gulf Arab states have long feared a sanctions-free Iran and as a nuclear agreement edges closer they have become more aggressive in preparing for a resurgence of their main regional foe. The biggest concern is what they see as Shi'ite Iran's regional expansionism, manifest in its backing for Syria's President Bashar al-Assad, Leba**n's Hezbollah, Iraq's Shi'ite militias and Yemen's Houthi group. From the airstrikes in Yemen to backing for Syrian rebels, *****downs on dissent and more forceful diplomacy, the actions of the mostly Sunni Muslim Gulf states in recent months has been driven by a sense that Iran is on the cusp of a comeback.