BEIRUT -- Iran will take part in international talks on Syria for the first time this week, giving it a voice in the effort to find a resolution to the more than 4-year-old civil war that has so far defied even the slightest progress toward peace.

A crucial backer of Syrian President Bashar Assad, Tehran has been shunned from all previous talks on Syria. Its inclusion now marks recognition by the United States that no discussion on Syria's future can succeed without Iran at the table.

News of Iran's attendance outraged Syrian rebels, who said its participation will only prolong the conflict.

The gathering, which takes place today and tomorrow in Vienna, will also put Iran in the same room with its most bitter regional rival, Saudi Arabia, raising the potential for tensions. The kingdom, along with other Gulf countries, has been funneling weapons to rebel factions, while Iran has sent financing, weapons and military advisers to ensure Assad's survival.

Iran's participation reflects its newfound place in the international community after the nuclear deal reached with world powers earlier this year. It also shows the seismic shift brought about by Russia's direct military involvement in Syria since launching a campaign of airstrikes on behalf of Assad last month. That intervention has emboldened Assad's supporters.

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Russia's intervention -- and its insistence that it seeks a political solution -- have created a new dynamic.

At the heart of the Vienna talks -- and the most contentious issue -- is the future of Assad.

While no one expects a breakthrough, the Vienna talks are the most serious attempt yet to put an end to a conflict that has killed a quarter of a million people and displaced millions of others, touching off a humanitarian crisis of spectacular proportions and unleashing Islamic extremists across the Middle East.