The Bedford Incident

The American destroyer USS Bedford (DLG-113) detects a Soviet submarine in the GIUK gap near the Greenland coast.[6] Although the U.S. and the Soviet Union are not at war, Captain Eric Finlander (Richard Widmark) harries his prey mercilessly while civilian photojournalist Ben Munceford (Sidney Poitier) and NATO naval advisor Commodore (and ex-Second World War U-boat captain) Wolfgang Schrepke (Eric Portman), look on with mounting alarm.
Because the submarine is not powered by a nuclear reactor, its submerged run distance is limited, critical when it also needs breathing air and to recharge its batteries. This gives Finlander an advantage but also means the Soviets will be more desperate. Also aboard the ship are Ensign Ralston (James MacArthur), an inexperienced young officer constantly being criticised by his captain for small errors, and Lieutenant Commander Chester Potter, USNR (Martin Balsam), the ship's new doctor, who is a reservist recently recalled to active duty.
Munceford is aboard in order to photograph life on a Navy destroyer, but his real interest is Captain Finlander, who was recently passed over for promotion to rear admiral. Munceford is curious whether a comment made by Finlander regarding the American intervention in Cuba is the reason for his nonpromotion, perhaps betraying veiled aggression. He is treated with mounting hostility by the captain because he is seen as a civilian putting his nose where it does not belong and because he disagrees with Finlander's decision to continue with an unnecessary and dangerous confrontation. Finlander is hostile to anyone who is not involved in the hunt, including the doctor, who will not stand up to the captain but advises that the pressure on the crew be reduced.
The crew becomes increasingly fatigued by the unrelenting pursuit during which the captain demands full attention to the instruments. When the submarine is found and ignores Captain Finlander's demand to surface and identify itself, Finlander escalates the situation by smashing into the submarine's snorkel, calling it "floating debris". Finlander then orders Bedford to arm weapons and withdraw a distance, where he will wait for the submarine's crew to run out of air and be forced to surface. He reassures Munceford and Schrepke that he is in command of the situation and that he will not fire first, but "If he fires one, I'll fire ONE!"
A tired Ensign Ralston mistakes Finlander's remark as the command to "fire one" and launches an anti-submarine rocket, which destroys the submarine. Sonar then detects four Soviet nuclear-armed torpedoes are targeting the destroyer. Finlander initially gives basic orders to evade but then silently steps outside. Munceford follows frantically pleading with the captain to do something.
Instead Finlander does nothing, knowing his actions have all but killed everyone on board the Bedford, as the ship cannot escape the nuclear torpedoes. The film ends with still shots of various crewmen "melting" as if the celluloid film were burning as the Bedford and her crew are vaporised in an atomic blast. The film's final image is an iconic, towering mushroom cloud.

Question: Who mistakes Finlander's remark as a command ?
Answer: Ralston.