Israel has dominated battlefields across the Middle East for the past two years, largely from the air. It has yet to lose a single plane. A major reason is the F-35. Now, a promise by the Trump administration to sell the most advanced U.S. aircraft to Saudi Arabia is threatening to remove a pillar of Israel’s uncontested military advantage over its neighbors. The immediate concern in Israel isn’t so much about Saudi Arabia’s potential use of the plane. Israeli officials are worried that sales to the kingdom would open the door to others, such as Turkey, that have made clear their aspirations for the stealthy and highly networked plane. Such sales would erode Israel’s advantage and put its own F-35s at greater risk at a time when the regional balance of power is shaped largely from the air...