WASHINGTON — A new fault line has emerged in Middle Eastern politics, one that will have profound implications for America’s foreign policy in the region. This rift is not defined by those who support or oppose the Islamic State in Iraq and Syria (ISIS), or by conflict between Sunnis and Shiites and the proxy war between Saudi Arabia and Iran. It is characterized by a fundamental division between Islamists and non-Islamists.

On one side are the Islamists — both Sunni and Shiite. ISIS and the Muslim Brotherhood represent the Sunni end of the spectrum, while the Islamic Republic of Iran and its militias, including Hezbollah (in Lebanon and Syria) and Asaib Ahl al-Haq (in Iraq), constitute the other. Many of these Islamists are at war with one another, but they are also engaged in a bitter struggle with non-Islamists to define the fundamental identity of the region and its states. What the Islamists all have in common is that they subordinate national identities to an Islamic identity.

To be sure, not all are as extreme as ISIS, which seeks to obliterate sovereign nations under the aegis of a caliphate. But the Muslim Brotherhood is committed to the Umma, the larger Muslim community. One reason behind the popular revolt against its rule in Egypt was that the Brotherhood violated a basic principle of national identity: It was Islamist before it was Egyptian.

Now, President Abdel Fattah el-Sisi sees his country engaged in an existential conflict with the Muslim Brotherhood. He is backed financially by Saudi Arabia, the United Arab Emirates and Kuwait. Mr. Sisi also collaborates closely with Algeria, and has support from Morocco and Jordan.