When oil prices started to edge down a year ago, most energy mavens thought the drop would be small and short-lived.

Instead, the price of crude has plunged by about 60% from its 2014 peak—and suddenly looks likely to stay low for months and maybe years to come. The reason: In the global battle for market share, nobody has backed down. Nobody has even blinked. Not Saudi Arabia, not the U.S., and not even troubled producers from Russia to Iraq. Everyone who can seems locked into pumping as much oil as possible.

...