In Russia’s shadow war with the west, one intelligence agency keeps making headlines.

The GRU, or main intelligence directorate, of the Russian army, has been accused of spearheading several of Russia’s most notorious operations in recent years. They include the 2014 seizure of Crimea using undercover soldiers called “little green men”, the hacking theft of emails from the Democratic National Committee and Hillary Clinton, and even the planning of a failed coup in Montenegro.

A British security source told the Guardian on Monday that the nerve agent attack on the former double agent Sergei Skripal was also ordered by the intelligence agency. The British government is poised to submit an extradition request to Moscow for two Russians suspected of carrying out the Salisbury attack that left one person dead and three injured, including Skripal and his daughter.

One of the three main Russian intelligence agencies, less has been written about the GRU (now officially called the GU, or Main Directorate of the General Staff of the Russian Armed Forces) than its sister agencies, particularly the Soviet-era KGB or its modern successor the FSB, the spy agency once headed by Vladimir Putin.

Known for operating under a wartime mentality and a willingness to take risks, experts say the culture of the GRU has been influenced by its inclusion of Spetsnaz special forces and experience in war zones, including Syria and Ukraine.

Some sources have linked GRU agents to the downing of Malaysian airlines flight MH17. Photograph: Antonio Bronic/Reuters

Open source researchers have claimed that a GRU officer supervised the transport of anti-aircraft weapons to eastern Ukraine when the Malaysian jetliner flight MH17 was shot down there, killing 298 people.

“The GRU regards itself as a war-fighting instrument. Yes, it gathers conventional intelligence … but its culture is much more military,” said Mark Galeotti, an expert on Russian security issues and the country’s intelligence agencies. “Although only a minority of GRU officers are Spetsnaz, it has an impact when part of your service are commandos.”

Besides special forces, the spy agency manages more traditional intelligence-gathering operations around the world, as well as signals intelligence.

Vladimir Rezun, a GRU officer who defected to the UK in the 1970s, wrote in his history of the agency (written under the pen name Viktor Suvorov) that it was largely tasked with preventing the collapse of the Soviet Union from without, as opposed to the KGB, which had a prominent role in thwarting internal threats.

While the KGB became notorious, the GRU largely operated in obscurity. “In the people’s consciousness, everything that is dark, underground and secret is connected with the KGB but not at all with the GRU,” he wrote.

Traditionally, Galeotti noted, the GRU answered for “uncontrolled spaces”. While in the past that has meant areas like civil wars, it may also apply to zones like cyberspace now.

US special counsel Robert Mueller recently linked the GRU to meddling in the 2016 US presidential election. Photograph: J Scott Applewhite/AP

Several Russian intelligence agencies were involved in hacking operations before the 2016 US presidential elections, but only the GRU was identified in an 11-count indictment released last month by the United States special counsel Robert Mueller’s investigation. The hack was perpetrated by staff employees of several Moscow-based units traditionally tied to signal intelligence.

The interests of Russia’s intelligence agencies regularly overlap, as have their methods. As Christopher Andrew wrote in The Mitrokhin Archive: The KGB in Europe and the West, his study of notes on KGB files smuggled out of Russia, the agency “offered its allies lethal nerve toxins and poisons which were fatal on contact with the skin for use during ‘special actions’”.

While the GRU has largely been shrouded in secrecy, there have been occasional contacts with the west.

Peter Zwack, a retired US army brigadier general, wrote about a series of meetings before the Sochi Olympics with the head of the GRU, Igor Sergun, who died unexpectedly of a heart attack in January 2016. “I found him soft-spoken, unassuming, complex, erudite and nuanced,” he said of their meetings, which largely focused on counter-terrorism efforts.

“I learned that even as Sergun relentlessly directed global intelligence operations against our interests, he — paradoxically — also viewed constant confrontation with the US and west as not in Russia’s best long-term interest,” Zwack wrote.

Those meetings ended after Russia’s invasion of Ukraine.