The Sentinel

Pete Garrison (Michael Douglas) is a Secret Service agent and one of the personal bodyguards for the First Lady of the United States Sarah Ballentine (Kim Basinger), with whom he is having an affair. He is one of the oldest and most experienced agents, having been involved in saving Ronald Reagan's life during the Reagan assassination attempt. A fellow agent and close friend, Charlie Merriweather (Clark Johnson), is later assassinated in front of his home. He gets wind from a trusted informant that the killing of Merriweather is related to an assassination plot against the President. The information provided by the informant revealed that a mole with access to the President's security detail had provided information to the assassins. The Secret Service Protective Intelligence Division, led by Garrison's former protégé and ex-friend David Breckinridge (Kiefer Sutherland) with rookie partner Jill Marin (Eva Longoria), is tasked with investigating the plot, which begins with polygraphs for every agent. Meanwhile, the mole discovers the discussion with the informant and Garrison's affair with the First Lady, and attempts to blackmail him by luring him to a coffee shop known to be a meeting point for Colombian gangs. Along with failing the polygraph test (for lying about the affair), Garrison becomes unwittingly framed by the mole and becomes the prime suspect for providing the information to the assassins.When Breckinridge confronts Garrison at his home and begins to interrogate him about his dealings with the plot, the source of the two's estrangement comes to light: Garrison supposedly had an affair with Breckinridges wife and caused the breakup of their marriage, which he denies. Garrison escapes capture and conducts an independent investigation of the assassination plot, while making brief contact with the First Lady to deny his involvement. He tries to contact the informant who gave him the tip, but finds that he has been killed. He briefly encounters Breckinridge while finding the informant, but Breckinridge refuses to kill him, despite giving other agents "shoot to kill" orders. Using his contacts with sympathetic agents and family members, he stumbles onto an apartment rented by one of the assassins, and finds information that the assassins are headed to Toronto to attack the president at the G8 summit. He tells Jill Marin of the discovery as he leaves but later finds that any incriminating evidence has been removed before she can arrive.Meanwhile, the First Lady discloses her affair with Garrison to Breckinridge, who catches up to Garrison in Toronto and tells him that he believes that Garrison is innocent. Together, they discover the identity of the assassins and the mole, senior agent William Montrose (Martin Donovan), who was never polygraphed. Montrose is in charge of directing security at the G8 summit. The leader of the assassins, a man known as the Handler (Ritchie Coster), tells Montrose to give him the President, to which Montrose refuses, not caring if he goes to prison for treason or if the Handler kills him. The Handler then states that should Montrose not cooperate, his children and wife will be killed, showing a photo of each of them, leaving Montrose with no choice. The Handler then says that all Montrose has to do is jam the communication radios and nothing more. He then leaves Montrose to make the sad decision as well as leaving him the photos, saying "You can keep those; I got doubles".Breckinridge and Garrison race to the summit, trying to whisk the President away to safety, with the assassins, dressed as RCMP ERT operators, trying to kill the President while the Handler, disguised as an RCMP officer, grabs the First Lady as a hostage. Several agents are killed, including Ed and Pamela. Montrose is killed by a fake ETF officer while in his last act of protecting the President, in spite of the threats against his family. Ultimately, Jill Marin and Breckinridge are able to successfully save the President while Garrison kills the Handler and saves the First Lady before the assassin leader can shoot anyone. However, Garrison is forced to take an early retirement due to the disclosure of the affair with the First Lady, who looks on sadly from her window as Garrison leaves the White House.

Question: Why is Garrison forced to take an early retirement ?