Russia, Turkey, and Iran agreed Tuesday on the outlines of a plan to reinforce a cease-fire in Syria, making the three most important outside players in the conflict as guarantors to a peace process. The deal, reached after two days of talks in Kazakhstan’s capital, Astana, drew Iran into an alliance with Russia and Turkey over ways to secure a settlement. The deal called for a cease-fire enforcement mechanism jointly monitored by the three states – which also committed themselves jointly to fight the Islamic State and Syria’s al-Qaeda affiliate. Observers noted that the deal would test Russia’s new role as a lead regional power broker.