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It's the regional heavyweight that few want at the table, but without it any attempt to end the Syria war may be futile. Iran's backing is crucial for President Bashar Assad's hold on power — and for the Iranians, Syria is key to their aspirations of regional power.
As an international conference on Syria kicked off Wednesday with the participation of more than 40 countries, Iran's absence hung over the meeting, following a diplomatic debacle that saw the U.N. withdraw a last-minute invitation after an uproar from the United States and the Syrian opposition.
The absence of Damascus' strongest regional ally stood out even more given that the biggest supporters of the opposition were all present: Saudi Arabia, Qatar and Turkey.
The question of Iran's participation underlines how the international powers that have lined up behind either Assad or the rebels trying to topple him are as crucial to a solution as Syria's warring parties themselves.
Like any of the regional players, Iran can be a spoiler for a resolution it opposes or can be a force for pressuring its side to make concessions.
"The decision to exclude Iran from the Montreux talks is a huge diplomatic mistake," said David Cortright, director of policy studies at the Kroc Institute for International Peace Studies.
"As a major backer of the current regime, Iran has enormous potential leverage in Damascus," he said.
However, he poured cold water on a U.N.-sponsored peace conference on the civil war inSyria, which opened in Montreux, Switzerland on Wednesday in the absence of Iran, a key ally of Damascus, which was excluded after refusing to endorse the goal of a transition from President Bashar al-Assad's rule.
"Reaching a conclusion will be very difficult but my wish is that if all efforts go in the direction of the wishes of the Syrian people, that is the direction that should prevail."
"As to whether this conference can achieve its objectives, I have a lot of doubt," he added. Rouhani accused Saudi Arabia and Qatar of backing "terrorists" in Syria in an effort to extend their own influence in the Middle East.