The main al-Qaeda affiliate in Syria is extending its control over a swath of territory that was until recently held by the collapsing moderate opposition, jeopardizing U.S. plans to form a new rebel force to fight extremists.

Since routing two of the biggest Western-backed rebel movements last month from the province of Idlib, Jabhat al-Nusra has been steadily consolidating its position as the single most powerful military force in northwestern Syria.

The group has overrun towns and villages throughout the province, secured supply routes into neighboring Turkey and potentially paved the way for the establishment of an Islamic “emirate” — a competing entity to the “caliphate” declared last summer by the Islamic State in northeastern Syria and western Iraq. […]