Hezbollah and Iran have been helping Assad’s forces to regain the momentum in the war, making it look more likely that he could ultimately prevail over the rebels. If and when the war ends, it’s increasingly plausible that Iran will emerge as the big winner, able to project even more influence in a weakened Syria and into Lebanon, where Hezbollah is based. “If Iran wins this conflict and the Syrian regime survives, Iran’s interventionist policy will become wider and its credibility will be enhanced,” an analyst at the Dubai-based Gulf Research Council told Sly.
... Long-held international relations theory maintains that states, though their leaders might nudge them a few degrees one way or another, tend to conduct foreign policy in accordance with their national interests. And the simple fact is that, for all the U.S. interests in Syria, Iran’s interests run deeper. The country just has much more to gain in “winning” the conflict than does the United States and much more to lose if it doesn’t.
It's certainly the case that Iran has more direct interests at stake in Syria than the US does – just as Iraq, Lebanon, Iraq and the Sunni Arab Gulf monarchies have more at stake. That's a function of proximity and, as Fisher rightly points out, Iran's dwindling number of friends in the region (see: Hamas). A rebel victory over Assad will certainly reduce Iran's regional leverage, make it harder to arm and support Lebanon's Shiite Hezbollah, and leave Iran even more isolated.
But when it comes to regional influence, any "victory" in Syria may well prove a Pyrrhic one. Just seven years ago, Iran and Hezbollah, along with the Sunni Palestinian Hamas, were aligned as an "axis of resistance" and feted across the Arab world for standing up to Israel and US interest in the region. Today, Hamas has distanced itself from Iran and both the Islamic Republic and Hezbollah are scorned in the region as butchers and worse for their involvement in Syria, in which they're fighting to preserve, in the view of most regional media, a secular autocrat and torturer of women and children.
While Iran's cordial relations with the Shiite-dominated government of Iraq, made possible by the US-led war to remove Saddam Hussein from power in 2003, may offset whatever losses they're already taking as a result of their support for Assad, the country's regional standing has already taken a meaningful longterm hit.
A few years ago, Iran could contract its own steadfast opposition to the US and Israel with the likes of close US partner Saudi Arabia, and find some resonance among Arab public's for its position. Today, heightened distrust of Iran is a given throughout the region. At the end of last month, influential Sunni preacher Yusuf al-Qaradawi, close to both the Qatari regime and to the Muslim Brotherhood that now rules Egypt, called for a jihad in Syria to push Hezbollah and Iran out of the country, and recanted his earlier kind words for Iran, saying he now sees the country as committed to wiping out Sunni Muslims.
Should the US join the war just because a defeat for Assad is bad for Iran's interests? Are US interests therefore automatically served? Not necessarily. The country could well have a long civil war after this war, the only certain outcome of which would be greater suffering. And in that event, the US would be involved in a multisided, dirty conflict far from home.