What is "fleets in being"

Usage examples of "fleets in being".

The arduous weather was claiming more casualties among his airmen than was the curiously negative response of the enemy, Yes, Operation sow was certainly putting to the test those years of peacetime training, those Goddam years of pinching and scraping to keep the fleets in being while Uncle Sam picked himself up again after the Vietnam syndrome, a malaise ably exploited by Soviet subversion throughout the free world.