The rapidly escalating conflict between Saudi Arabia and Iran, sparked by the execution of a Saudi Shiite activist, may seem like the natural outgrowth of a decade's Sunni-Shiite tensions. But more than denominational differences, what's driving the open conflict is the Saudis' deepening fear that the U.S. is shifting its loyalties in the Persian Gulf region from its traditional Saudi ally to a gradually moderating Iran. And in a sense, they're right: Although the U.S. is a long way from becoming an instinctive Iranian ally, the nuclear deal has led Washington to start broadening its base in the Gulf, working with Iran where the two sides have overlapping interests. Of which there are many these days.

The Saudis executed the activist, Nimr al-Nimr, last weekend because they wanted to send a message to the country's Shiite minority and neighbors, and because they thought they could get away with it. The outspoken al-Nimr symbolized the possibility that Saudi Shiites might never fully accept their second-class status and, worse, might seek autonomy or independence in the event of the Saudi state's weakness. The Saudis seem to have calculated that if Iran made any noise about the execution, it would not have leverage to do anything about it. Undoubtedly the Saudis knew the Americans wouldn't be best pleased with them for killing a nonviolent activist — but again, they must have thought it wouldn't matter.

Executing al-Nimr was thus probably intended to demonstrate that the Saudis can go it alone, making security-related decisions without worrying what their neighbors or the U.S. think. If that's right, the execution was an indirect signal that Riyadh is feeling isolated, and that if isolated, it will act unilaterally.