Crimson Tide

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In post-Soviet Russia, a rebellion originating in Chechnya prompts bombing strikes by Russia. The United States, the United Kingdom and France suspend all aid to Russia in response. Russian Ultranationalist leader Vladimir Radchenko denounces this as an act of war, and calls for revolt. Martial law is declared in Russia as forces loyal to Radchenko seize a region around Vladivostok, including a naval base and a nuclear missile base.
A U.S. Navy ballistic missile submarine, the USSÂ Alabama, commanded by combat veteran Captain Frank Ramsey (Hackman), is assigned to patrol in the Pacific Ocean to be available to launch a pre-emptive nuclear strike. He chooses Lieutenant Commander Ron Hunter (Washington) as his executive officer, an officer with an extensive military education but no combat experience, as his XO has appendicitis.
During their initial days at sea, differing personalities cause tension to rise between Ramsey and Hunter; Hunter is more analytical and cautious, while Ramsey is more intuitive, impulsive, and demanding. Ramsey orders a mock missile launch drill while Hunter is tending to an out-of-control fire in the ship's galley, which delays his response to the drill. Afterward, Ramsey explains to Hunter that chaos is the perfect opportunity to test the crew's abilities, while Hunter privately feels the action was irresponsible and coincidentally resulted in the demise of a crewmember who suffered an MI.
Hunter believes crew morale will suffer from the situation and constant testing, and observes two crewmen fighting over a seemingly trivial matter. Hunter informs Ramsey but he is unconcerned, and embarrasses Hunter over the ship-wide intercom and tells the crew to stay focused on the mission.
Alabama eventually receives an Emergency Action Message, that Russian nuclear missiles are being fuelled, and ordering the launch of ten of its missiles against the nuclear installation. Before Alabama can launch its missiles a second message arrives, but the radio equipment is damaged by the attack of a Russian Akula-class submarine loyal to Radchenko, revealing only a part of the message.
With the last confirmed order being to launch their nuclear arsenal, Capt. Ramsey decides to proceed. Hunter refuses to concurâhis concurrence is procedurally required to launchâbelieving the second message may be a retraction. Hunter argues that other submarines will launch their missiles if Alabama does not, as part of the Navy's standard redundancy doctrine. Ramsey argues that the other American submarines may have been destroyed, as theirs nearly was, leaving them the only ones to comply with the order to launch.
When Hunter refuses to concur with the launch order, Ramsey tries to relieve him of duty and to replace him with a different officer. Instead, Hunter orders the arrest of Ramsey for attempting to circumvent regulations, and is supported by COB, the Chief of the Boat. The crew's loyalty is divided between Hunter and Ramsey, but Hunter eventually takes command. Other crewmen try to repair the radio while the battle for command ensues.
Alabama is attacked again by the Russian submarine. Alabama evades two torpedoes and destroys the Akula, but not before it fires a third, which explodes near Alabama, flooding the bilge bay and disabling the submarine's main propulsion. Alabama begins to sink.
The crew tries desperately to repair the damage. COB warns Hunter that the bilge bay must be sealed or the ship will be lost. Hunter orders the crewmen to exit the bilge bay and seal it. Unfortunately, they are unable to exit the bilge bay in time, and the compartment is sealed with them inside. Shortly before the Alabama reaches crush depth, the propulsion system is repaired in time and Hunter orders the sub to ascend to periscope depth.
In the meantime, Lieutenant Dougherty, who is loyal to Ramsey, organizes a mutiny against Hunter with Lieutenants Zimmer and Westergard, and tries to recruit Lieutenant "Weps" Ince. In A long-time friend of Hunter, Ince is hesitant, but through force of will, joins the mutiny. Ramsey and the mutineers draw small arms and recover the control room from Hunter, confining Hunter and his supporters. Ramsey then proceeds with the missile launch order, and engages the missile lock key at the bridge.
Hunter and supporters escape their confines and gain the support of Ince in the missile control room, further delaying the launch. Ramsey leaves the control room and tries to force Ince to open the safe containing the firing trigger. Ramsey threatens to shoot Ince, but since only Ince knows the combination to the safe, threatens to shoot a Petty Officer of Ince's team. Ince relents, and opens the safe.
Meanwhile, Hunter and his supporters retake the control room. Hunter deactivates the missile system just as Ramsey pulls the firing trigger. Ramsey returns to the control room and demands Hunter hand over the missile key, but Hunter refuses. With the radio team reporting their repairs almost complete, the two men agree to wait until the launch deadline to see if the radio can be repaired. After several tense minutes, communications are restored and they finally see the full message of the second transmission:it is an order to abort the launch because Radchenko's rebellion has been quelled. A chastened Ramsey leaves the control room.
After returning to base, Capt. Ramsey and LCDR Hunter are put before a board of inquiry. The panel concludes that both men were simultaneously right and wrong, while unofficially chastising both men for failing to resolve their differences. Thanks to Capt. Ramsey's personal recommendation, the panel recommends LCDR Hunter be given command at the earliest possible convenience, and that Capt. Ramsey be permitted to retire early. The men reconcile, and part ways.

Question: Why does Ramsey try to relieve Hunter of his duty and replace him with a different officer?
Answer: Refuses to concur with the launch order