Drones have come of age in the war on terror. When the United States and Britain invaded Afghanistan in 2001, the technology was barely out of the lab. Today, these flying machines represent a huge security threat. If reports are to be believed, a Houthi rebel-launched drone attack in Saudi Arabia last weekend shut down 5 per cent of the world’s global oil supply and caused the largest spike in the price of oil since the first Gulf War in 1990.