Saudi Arabia has one of the world's most expensive and sophisticated arsenals, filled with American warplanes, missile defenses and other cutting-edge technology.

The distinction between supporting Saudi forces and fighting alongside them may become increasingly difficult to maintain, says Derek Grossman of the University of Southern California.

Riyadh airport will remain closed at least until the weekend amid deadly strikes by Yemen-based Houthis. The US has said it is weighing joining Saudi military operations.