WASHINGTON — 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.

Even with American intelligence providing the latest in surveillance, the Saudi military has often been unable to act effectively, reinforcing a view among national security officials and humanitarian activists that — despite all the sparkling, expensive hardware — Saudi Arabia remains uninterested or incapable of defending its entire territory or competently and humanely prosecuting a war abroad.