WASHINGTON — As the United States and Iran continue efforts to reach a long-term nuclear agreement, it is becoming increasingly evident that they are tacitly aligned in a longer-term struggle against the mutual threat of Sunni extremism.

US Defense Secretary Ashton Carter indirectly confirmed July 7 that the “boots on the ground” battling the group that calls itself the Islamic State (IS) are primarily forces also backed by Iran, including Shiites and Kurds.

The United States has managed to train only 60 vetted individuals to fight IS in Syria, Carter told the Senate Armed Services Committee. The figures for Iraq are better but hardly impressive. As of June 30, Carter said, the United States has trained fewer than 11,000 Iraqis — of whom only 1,300 are Sunnis — to join forces seeking to roll back IS advances in Iraq.

There are ample reasons for this poor showing. Many Syrians want to overthrow the Iran-backed government of President Bashar al-Assad more than they want to defeat IS. Iraqi Sunnis are also unenthusiastic about risking their lives in support of another Iran-backed government in Baghdad.