MOSCOW—Since the breakup of the Soviet Union, Russia’s military intelligence agency has been blamed for what Western experts say are some of the Kremlin’s most nefarious activities, destabilizing neighbors and embarking on information warfare against enemies and allies alike.

This week, U.S. special counsel Robert Mueller accused 12 officers in Russia’s military intelligence, known formally as the Main Intelligence Directorate, or by its acronym GRU in Russia, of hacking the Democratic National Committee’s servers and spreading the stolen information.

For decades, the agency has operated in the shadows, fiercely independent of Russia’s many other intelligence agencies, like the Federal Security Service, or FSB, the successor agency to the KGB. But GRU’s fingerprints have been found wherever the Kremlin has employed the dark arts of espionage and subversion, analysts, diplomats and Western intelligence officials say.

“They are the armed forces of Russia and the intelligence apparatus that does reconnaissance, surveillance and apparently strategic cyber operations, which they have for years,” said Malcolm Nance, a career U.S. naval intelligence officer who specializes in cryptology, national security policy and counterterrorism intelligence.

Under Russian President Vladimir Putin, the GRU has been instrumental in helping the Kremlin achieve its foreign objectives of boosting regional and global influence, experts say.