John Carter

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In 1881, Edgar Rice Burroughs attends the funeral of his uncle, John Carter, a former American Civil War Confederate Army captain, who died suddenly. The body is put in a tomb unlockable only from the inside. The attorney executing the will hands Burroughs Carter's journal, which he reads, hoping to find clues about Carter's death and the reason he is the willed heir. The bulk of the film enacts what Burroughs reads.
While prospecting in the Arizona Territory in 1868, just after the Civil War, Carter is arrested by Union Colonel Powell, who seeks his help in fighting the Apache and insists that Carter owes it to his country. Carter refuses, stating that any debt was paid when he lost his family. Carter escapes from his cell but is pursued by Powell and his cavalry. After encountering a band of Apaches, Carter and the wounded Powell take refuge in a cave, which turns out to be the object of Carter's earlier search: the "Spider Cave of Gold". A mysterious priestly being, a Thern, appears in the cave at that moment, and surprised by Carter's presence, immediately attacks him with a knife; Carter kills him but accidentally activates the Thern's powerful medallion, and is unwittingly transported to the ruined and dying planet of Barsoom, later revealed to be Mars.
Due to Barsoom's low gravity and his own relatively high bone density, Carter is able to perform incredible jumps and feats of strength. He is captured by the Tharks, a clan of four-armed Green Martians led by their "Jeddak" (chieftain), Tars Tarkas. Tars instructs Sola to watch over Carter, leading to her feeding him a liquid that enables him to understand the Martians' language.
Elsewhere on Barsoom, the human Red Martian city of Helium, led by Thardos Mors, and the Walking City of Zodanga, led by the villainous Sab Than (Dominic West), have been at war for a thousand years. Sab Than, who wants to conquer all Barsoom, is armed with a special and highly destructive weapon given him by three Therns in the midst of a ship-to-ship close quarters combat, which the weapon ends by annihilating all the other participants. Sab Than proposes a cease-fire with Helium and an end to the war, sealed by marriage to Mors' daughter, the Princess of Helium Dejah Thoris. The unwilling Princess, disguised as a soldier, escapes in a Helium ship.
As Tars Tarkas tries to get John Carter to show off his jumping abilities, a Thark sentry spots a ship from Helium and another from Zodanga, and the Tharks scatter to hide. Carter races into action and leaps to save the human Dejah from falling. He kills some Zodangan soldiers and briefly fights with Sab Than. Than's ship retreats and John Carter is hailed by Tars Tarkas as "Dotar Sojat" (roughly translatable as "My Right Arms"), for his strength and skill. Tarkas even grants him Dejah as part of the Thark spoils. Later, Carter, accompanied by Dejah, tries to find a way to get back to Earth, and stumbles upon a temple ruin sacred to the Tharks; Sola encounters them there and vainly tries to stop them from entering. They discover an inscription depicting a way to Earth in the sacred river of Iss, before Carter, Dejah, and Sola are caught by Sarkoja and Tal Hajus. The three are sentenced to death according to the Thark code, but are helped to escape by Tars Tarkas, who reveals to Carter that Sola is his daughter. Tal Hajus and Sarkoja find the prisoners gone, and the ambitious Hajus declares that Tarkas has betrayed them.
Carter, Dejah, Sola, and Woola (a "Calot" - a large creature that resembles an amphibian and behaves like a loyal dog) embark on a quest to end of the sacred river to find a way home for Carter. They discover clues to the "ninth ray", a means of accessing infinite energy and the key to understanding how the medallion works. They are attacked by the Green Martian Clan of Warhoon, who have been manipulated by Matai Shang to pursue them as part of Sab Than's latest plan. After initially fleeing, Carter sends the others away and takes a stand, fighting the horde himself as atonement for being unable to save his own family. Though defeating many Warhoon, Carter is ultimately overpowered; he is saved when Thardos Mors intervenes in a Helium ship. Sab Than is also in the company: Mors says that Than had come alone and had organized the rescue party. Dejah reluctantly agrees to marry Sab Than and Carter is taken to Zodanga to be healed.
Upon awakening, Carter is led to Dejah's room. Dejah sends the servant girls away and gives Carter his medallion, telling him to return to Earth. As Dejah leaves with Sab Than, Carter is met by Matai Shang, who takes Carter for a walk around Zodanga. In different Zodangan forms, Shang explains to Carter the Therns' purpose and how they manipulate the civilizations of various planets into self-destruction. He also reveals Sab Than's secret plan to kill Dejah once he marries her and to destroy Helium and rule Barsoom, so completing the course the Therns have set for the planet. (Shang also reveals that he and the Therns had been doing this for millions of years.) Carter escapes thanks to Woola and he and Sola return to the Tharks to request their help. There, they discover Tal Hajus has overthrown Tars Tarkas. Tarkas, Carter, and Sola are thrown into an arena to be killed by two enormous four-armed Great White-Apes. Carter defeats them and then challenges and easily kills Hajus, so becoming Jeddak of the Tharks.
Carter and his Thark army charge on Helium and defeat the Zodangan army in a huge battle. Sab Than is killed by a Thern before revealing anything more about them to Carter. Dejah and Carter marry and he becomes Prince of Helium. On their first night, Carter decides to stay forever on Mars and throws away his medallion. Seizing this opportunity, Matai Shang, in the form of a Helium guard, sends him back to Earth, before leaving Mars forever. (Shang is not seen again after this scene.)
Back on Earth, Carter concludes that there may be other Thern 'Way-stations' like the 'Spider Cave of Gold' and embarks on a long quest for clues of the Therns' presence on Earth and hoping to find one of their medallions; after ten years he appears to die suddenly and requests his unusual funeral arrangements â consistent with his having found a medallion, since his return to Mars would leave his Earth body vulnerable in a coma-like state. He leaves Burroughs clues about opening the tomb.
The film reverts to the original timeline. Burroughs races to Carter's tomb and opens it, expecting to find Carter's body. A Thern, in the form of a man with a bowler hat(who has been shadowing Carter for some time), appears behind Burroughs holding a weapon with which to destroy the body. Both are taken aback to see the tomb empty. A shot rings out and the Thern drops dead. Carter emerges and confesses to Burroughs that he never found a medallion. Instead, he had devised a scheme to lure a Thern in order to get its medallion. Carter reiterates to Burroughs that his body on earth needs protecting &amp; recommends that Burroughs enjoy his life on Earth, to fall in love and to write books (affirming the conceit that the Burroughs character is the real-life author of the Barsoom novels). He then takes the Thern's medallion, whispers the code, and is transported back to Barsoom and Dejah.

Question: Who are they attacked by?
Answer: Green Martian Clan of Warhoon