Facing a host of military threats, the oil-rich monarchies of the Persian Gulf have been boosting outlays for defense equipment—providing a shot in the arm for Western arms suppliers facing government cutbacks in much of the rest of the world.

Lower oil prices may eventually damp the buying binge, as they did during a price slump in 1999. But for now, weapon sales to Middle East buyers, chief among them Saudi Arabia, are booming.

Riyadh spent $80.8 billion on defense last year, up almost 43% from just two years earlier and trailing only the U.S. and China, according to the International Institute for Strategic Studies. The four biggest Persian Gulf defense customers—Saudi Arabia, the United Arab Emirates, Oman and Qatar—spent a total $109.9 billion in 2014, up from almost 44% from 2012. […]