But he offered a warning to the West now commonly heard among fighters seeking the overthrow of President Bashar al-Assad: The Syrian people are being radicalized by a combination of a grinding conflict and their belief that they have been abandoned by a watching world. …

This is a theme that has resonated in recent days, not just in Syria, but in Turkey, where the government fired artillery shells into northern Syria this week after a Syrian mortar round hit a Turkish town and killed five civilians. In Turkey, there is a growing sense of frustration shared by the Syrian rebels that the West, the United States in particular, called for Mr. Assad to leave power, only to sit quietly on the sidelines as the crisis transformed into a bloody civil war. …

Across northern Syria, in areas that rebels have wrested from government control, such sentiments have become an angry and routine element of the public discourse. Wearied by violence, heading into another winter of fighting, and enraged by what they see as the inaction and hypocrisy of powerful nations, frontline leaders of the rebellion say that the West risks losing a potential ally in the Middle East if the Assad government should fall.

The corollary is frequently sounded, too: The West may be gaining enemies where it might have found friends. As anger grows, armed groups opposed to the United States may grow in numbers and stature, too.