For the US, how to deal with Assad without striking the Russians is a key issue – as is dealing with Syria's neighbors to consolidate spheres of influence.
Rouhani’s opponents – who see him as a threat to many of their interests – are targeting a struggling economy and the nuclear deal in bid to unseat him in 2017.
Threatening encounters between the US and Iran have risen recently in the Persian Gulf, with at least 32 reported so far this year, compared with 23 in all of 2015.
At a charity foundation south of Tehran, hundreds of donated sheep are slaughtered for Eid al-Adha, a feast that recalls God's test of Abraham's faith.
Russia's use of a base inside Iran to launch strikes in Syria marks unprecedented cooperation between the Islamic Republic and a foreign power.
To sell Iranians on the nuclear deal, President Rouhani promised a new era. But the consensus is it has yet to materialize, and many blame the US.
Iran opened a recruiting center in Herat, Afghanistan, last fall and has persuaded – and sometimes coerced – thousands of Afghans to fight in Syria.
Tensions between the longtime rivals soared after Saudi Arabia executed a prominent Saudi Shiite cleric on Jan. 2. The two countries also support opposing sides in Syria's civil war.
Such a shift has the potential to reshape the country’s regional profile – though it is unclear if would play a productive role or expand its proxy wars against Iran.
Turkey and Iran are engineering a partial rapprochement after an extended period of heated disputes. Turkey is hosting the Organization of Islamic Cooperation this week.