For the past half-century, the United States has trained and supplied the Saudi military, selling the wealthy kingdom more than $150 billion in dazzling high-technology weapons, including fighter jets and air defense systems.

And yet, the kingdom could not protect a prized national asset — its oil installations — from a recent attack by low-flying cruise missiles and drones that caused the largest rise in crude oil prices in a single day. The advanced weapons the United States sold to the Saudis include the Patriot air-defense system, but it is deployed near important military installations, and not oil infrastructure.

Nor has the country’s military managed to defeat Iranian-backed Houthi insurgents in Yemen, despite a four-year, Saudi-led bombing campaign that has left more than 8,500 civilians dead and more than 9,600 injured, according to international monitors…

During one episode three years ago, American intelligence officials gave their Saudi counterparts the location of 30 Houthi insurgents who had crossed the border and entered the kingdom. But the Saudis were unable to muster anyone to go after them, according to a former senior Defense Department official. The insurgents stayed in Saudi territory for eight hours — a period that included an extended rest break — all the while tracked by American intelligence before returning to Yemen, officials said.