The Guardian Office was the second organization within the Church of Scientology whose purpose was to protect the Church from outside influences. It had an intelligence gathering arm, as well as public relations, finance and legal arms. It existed from 1966 to 1983. It was headquarted at Saint Hill Manor, England.
Operation Snow White
Operation Snow White was the GO's most extensive and detailed operation. It was a far reaching investigation and espionage operation planned out against various hostile governments across the world. Its primary purpose was to reign in the proliferation of false reports on Scientology and the Church, so L. Ron Hubbard and the Church's management headquarters and peak service delivery organisaation abourd the ship Apollo could dock and conduct business without hinderance.  Projects were begun and conducted on countries ranging from Algeria to the UN.
The FBI Raids
In 1976, GO's effort were intercepted by the FBI. After collecting sufficient evidence, on July 7th, 1977 the FBI raided the GO's offices in Washington D.C. and Los Angelas, confiscating 48,149 documents dealing with Operation Snow White.  Mary Sue Hubbard, and 11 other top Church officials of the GO were arrested and served with indictments for conspiracy.
Fall of the Guardian Office
The dissolution of the Guardian Office mostly came about due to internal politics of the Church. After the FBI raid of the GO's locations in the late 70's, Hubbard lost his faith in the division, and particuarly his then wife Mary Sue. However, the GO was a huge division in the Church, numbering 1100 strong, and either directly ran or oversaw much of the Church's operations.
To counter the influence of the GO, Hubbard formed the Commodore's Messenger Organisation International, comprised mostly of his personal Sea Org assistants. Placed in charge of the new organization was David Miscavige, who still wields a large influence in the Church today, particuarly after Hubbard's death. As the conduit through which all of Hubbard's directives and communications went through, the CMO held the reins on all the material other divisions relied upon. As personal executors of Hubbard's wishes, the CMO had a large amount of clout with rank and file Scientology members, and had the personal authority that the GO lacked after the raids. (The preceeding paragraph is in dispute. The CMO existed much earlier than indicated.)
In 1981, as Mary Sue Hubbard approached the end of her appeals process, a contentious meeting was held between her and Miscavige, in which Mary Sue stepped down as head of the GO. Systematically those ranking GO officers who were not spending jail time were either forced out or resigned from their post. In their place were put Sea Org represenatives. By 1983 the remnants of the Guardian Office were transferred under the new title Office of Special Affairs. Unlike the GO however, the OSA was part of the thumb of Sea Org, assuring that no Scientology org or division would ever challenge the prominence of the Sea Org.