Turkey and Iran, regional heavyweights and heirs to imperial pasts, expanded trade in the past decade and papered over their traditional rivalry with diplomacy and rhetoric. Now these neighbors have staked out opposing positions in Syria, where outside players seek to sway an outcome to the bloodshed that could, in turn, alter power balances in the Middle East.Iranian-Turkish tension could grow if regional efforts to end the violence intensify as expected after Russia and China vetoed a U.N. resolution calling for the ouster of Syrian President Bashar Assad. Ankara wants him to leave, while Tehran supports him. At the same time, they want to preserve warm ties that mask fundamentally different tactics and visions.Turkey\'s willingness to clash over Syria is likely to be tempered by reliance on Iran for one-third of its oil supplies, as well as natural gas, that have helped to power its impressive economic engine. The Turks have also sought to make mediation a centerpiece of foreign policy, and that includes hopes for a diplomatic solution to the dispute over Iran\'s nuclear program.Turkey, NATO\'s biggest Muslim member, hosts the Syrian opposition and has compared Assad, a former ally, to Slobodan Milosevic, the ousted Serbian leader whose war crimes trial was interrupted by his death. Turkey\'s foreign minister, Ahmet Davutoglu, is in the United States this week to coordinate efforts to isolate the Syrian regime.Even before the protests in Syria, Turkish economic investments and other involvement there countered Iranian influence, said Savas Genc, an associate professor of international relations at Istanbul\'s Fatih University. Now with the region in turmoil, the two sides will struggle to maintain their \"constrained friendship,\" he wrote in an email.