GRU  is in the military intelligence service of the Russian Federation, (formerly the Red Army of the Soviet Union).

"GRU" is the English version of the Russian acronym ГРУ, which means Main Intelligence Directorate.

The GRU is Russia's largest foreign intelligence agency. In 1997 it had six times as many agents in foreign countries as the SVR (The SVR is the successor to the KGB's foreign operations directorate). The GRU commanded 25,000 Spetsnaz ('special') troops in 1997.

History 
The GRU's first predecessor in post-tsarist Russia was created on October 21, 1918 under the sponsorship of Leon Trotsky, who was then the civilian overseer of the Red Army. It was originally known as the Registration Directorate (Registrupravlenie, or RU). In his history of the early years of the GRU, Raymond W. Leonard writes:"As originally established, the Registration Department was not directly subordinate to the General Staff [of] the Red Army... In 1921... it was elevated in status to become the Second (Intelligence) Directorate of the Red Army Staff.

It was given the task of handling all military intelligence, particularly the collection of intelligence of military or political significance from sources outside the Soviet Union.  The GRU operated residencies all over the world. They ran with the SIGINT (signals intelligence) in Cuba, and throughout the former Soviet bloc countries, especially in Lithuania, Latvia, and Estonia.

The GRU was well known in the Soviet government for its fierce independence from the rival "internal intelligence organizations", such as NKVD and KGB.  At the time of the GRU's creation, Lenin infuriated the Cheka (predecessor of the KGB) by ordering it not to interfere with the GRU's operations.  Nonetheless, the Cheka infiltrated the GRU in 1919.  This planted the seed for a fierce rivalry between the two agencies, which were both engaged in espionage. The rivalry was even more intense than that between the FBI and CIA.

The existence of the GRU was not publicized during the Soviet era. Documents about it became available in the West in the late 1920s, and it was mentioned in the 1931 memoirs of the first OGPU defector, Georges Agabekov. Walter Krivitsky, the most senior Red Army intelligence officer ever to defect, described it in detail in his 1939 autobiography (I was Stalin's agent).

It became widely known in Russia, and the West  during perestroika. "Viktor Suvorov" (Vladimir Rezun), a GRU agent who defected to Great Britain in 1978, wrote about his experiences in the Soviet military and intelligence services.  According to Suvorov, even the General Secretary of the CPSU could not enter GRU headquarters without going through a security screening.

The GRU is still a very important part of the Russian Federation's intelligence services, especially since it was never split up like the KGB.  The KGB was dissolved after aiding a failed coup in 1991 against the then Soviet President Mikhail Gorbachev. It has since been divided into the Foreign Intelligence Service (SVR) and the Federal Security Service (FSB).

The GRU was officially dissolved on 7 May 1992. It continues as the Main Intelligence Directorate of the Russian Armed Forces.

References 

Soviet Union
Russia
Intelligence agencies
1918 establishments in the Soviet Union
1991 disestablishments in the Soviet Union