The film is set in Korea during the Korean War. Bogart (in his sole film for Metro-Goldwyn-Mayer) plays a surgeon and commander of Mobile Army Surgical Hospital (MASH) 8666 (shortened to "66" in the dialogue), with Allyson playing a newly arrived nurse. Despite their initial handicaps, their love flourishes against a background of war, enemy attacks, death and injury.
A young Army nurse, Lieutenant Ruth McGara (June Allyson), is newly assigned to the 8666th MASH, a mobile field hospital constantly on the move during the Korean War. At first, Ruth is a bumbling addition to the nurse corps, but attracts the attention of the unit's hard-drinking, no-nonsense chief surgeon Dr. and Major Jed Webbe (Humphrey Bogart). Jed cautions that he wants a "no strings" relationship and Ruth is warned by the other nurses of his womanizing ways. She sees that he is beloved by the unit, especially the resourceful Sergeant Orvil Statt (Keenan Wynn).
Three incidents punctuate their time at the front. When a young Korean child needs special care, Ruth entreats the chief surgeon to perform an open-heart operation, despite the reservations of the unit commander, Colonel Hilary Whalters (Robert Keith). Yet Jed is a relentless taskmaster, demanding Captain John Rustford (William Campbell) fly desperately needed blood supplies at night, even in the teeth of a fierce storm. When the helicopter lands safely, Jed goes on a binge, forcing Whalters to make his chief surgeon either straighten up or ship out. When a now more assured Ruth treats some North Korean prisoners, a frightened prisoner with a concealed grenade is calmly disarmed by her soothing words.
After a North Korean advance forces the MASH unit to escape through enemy lines, the lovers are temporarily separated. When the unit commander is wounded in an attack, Jed has to assume leadership to lead the unit out of danger. Traveling cross-country, he sets out on a perilous journey, attempting to meet up with the nurses who have gone on ahead. Eventually the two caravans safely negotiate the battlefield, and Jed and Ruth are reunited.
As appearing in Battle Circus, (main roles and screen credits identified):Humphrey Bogart as Major Jed Webbe
June Allyson as Lieutenant Ruth McGara
Keenan Wynn as Sergeant Orvil Statt
Robert Keith as Lieutenant Colonel Hilary Whalters
William Campbell as Captain John Rustford
Perry Sheehan as Lieutenant Laurence
Patricia Tiernan as Lieutenant Rose Ashland
Adele Longmire as Lieutenant Jane Franklin
Jonathan Cott as Adjutant
Ann Morrison as Lieutenant Edith Edwards
Helen Winston as Lieutenant Graciano
Sarah Selby as Captain Dobbs
Danny Chang as Korean child
Philip Ahn as Korean prisoner
Steve Forrest as Sergeant
According to Richard Brooks (in an interview filmed for the 1988 Bacall on Bogart documentary), Battle Circus was originally called MASH 66, a title rejected by MGM because the studio thought people would not understand the connection to a military hospital. The title of the film actually refers to the speed and ease with which a MASH unit, with its assemblage of tents, and portable equipment, can, like a circus, pick up stakes and move to where the action is. The film's technical advisor, Col. K. E. Van Buskirk had commanded one of the first MASH units in Korea, and ensured that the MASH and aerial scenes were authentic.
Brooks also noted that Bogart agreed to do the film because the script's humor, set in a story showing the tragedy of war, would make the film seem more realistic to an audience. Bogart was teamed with an old pal, Keenan Wynn and June Allyson who was also a friend. Allyson was initially afraid of acting with Bogart, but her recent roles in light romantic comedy had typecast her and she was encouraged by the studio to attempt more serious fare.
In retrospect, Bogart did not enjoy working on the film, the least of which was when he burnt his left thumb, a scene that was left in the final production. His $250,000 fee was the sole compensation. Bogart told Brooks, a close friend, "let's not make any more movies together."
Principal photography took place on location in Calabasas, California, and Camp Pickett, Virginia, where MASH units trained. The aerial sequences with the Bell 47 helicopters were filmed there. The camp commandant offered the film crews use of the base facilities, including his house for the lead actors, after initial scenes were finished.
According to MGM records the film earned $1,627,000 in the US and Canada and $735,000 overseas resulting in a profit of $265,000.
While commending Battle Circus for a revealing and engrossing wartime drama, reviewers noted that the tepid love story distracted. The review in The New York Times noted, "Unfortunately for the general pace and impact, considerable time is allotted to a dawdling and familiar personal drama, the romance of an Army surgeon and a rookie nurse."