REYHANLI, Turkey—Many Syrian rebels blame the opposition's loss of the vital city of Homs this past week on sheiks and their small militias who have drawn funding and arms away from more-experienced antigovernment commanders.

For the past two years, these religious leaders, who also oppose President Bashar al-Assad's regime but have little battlefield savvy, have attracted most of the funding from the Sunni-led Gulf States that are the rebellion's main underwriters.

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