Hundreds of Russian diplomatic staff have been expelled by European countries since March.Russia’s attack on Ukraine prompted the expulsions, but many of those expelled are suspected of spying.European countries have waged an intensifying battle against Russian espionage in recent years.LoadingSomething is loading.Amid the war in Ukraine, a clandestine battle is being waged throughout Europe.Since the beginning of March, more than 400 Russian diplomatic staff have been expelled from embassies and diplomatic missions across the continent, including in most EU countries.The majority of those expelled are suspected of being Russian intelligence operatives.Major expulsions of diplomatic staff have happened before. European countries expelled nearly 200 Russians after Russian intelligence operatives attempted to assassinate defected spy Sergei Skripal in the UK in 2018.However, this wave of expulsions is the biggest since the fall of the Soviet Union, and it may be the biggest in history.Everyone is in on itBanners left by Ukrainians during protests against Russia’s invasion of Ukraine outside the Russian consulate in Krakow, Poland, March 23, 2022.Omar Marques/Getty ImagesThe current wave is notable for the number of Russians expelled and for the number of countries expelling them: 23 EU countries, three non-EU European countries, and the EU itself.Chris Miller, a professor at the Fletcher School of Law and Diplomacy at Tufts University, called the expulsions “a long overdue response to Russia’s security services’ activity in Europe.””For too long, many European governments have given them a free hand, even though Russian agents have been implicated in assassinations and attacks from Berlin to London,” Miller, who is also director for Eurasia at the Foreign Policy Research Institute, told Insider.Underscoring the move’s significance is the fact that several of the countries doing the expulsions — including Austria, Bulgaria, and Germany — have good relations with Moscow. Some of the expelled staff were legitimate diplomats, like the Russian ambassador to Lithuania, whose expulsion was linked to the Russian invasion of Ukraine.Nevertheless, the majority of those expelled were suspected of being intelligence operatives or diplomats acting in contravention of the Vienna Convention on Diplomatic Relations, which governs diplomatic relations between countries.Poland expelled 45 Russian diplomats on March 23, which a spokesman for the country’s intelligence chief justified by saying, “Russia uses diplomacy not to remain in contact with partners, but to push false claims and false propaganda statements against the West.”A wide web of activitiesSurveillance footage of the two men suspected of carrying out the attack on Sergei Skripal, in Salisbury.London Metropolitan PolicePushing false claims, otherwise known as disinformation, has been a staple of Russian activity in Europe for decades. By doing so, Moscow aims to promote its interests and erode trust in foreign governments and institutions.Russian agents in a target country increase the reach of disinformation by approaching and collaborating with locals who amplify it.Russian interference was detected in the 2017 French presidential and German federal elections, in calls for Catalonia to secede from Spain, and in Scotland’s 2014 independence referendum. Some suspect Russia meddled in the 2016 Brexit vote to advance Moscow’s goal of undermining the EU.Russian espionage also targets NATO and the EU as organizations. In October, NATO expelled eight “undeclared” intelligence agents from Russia’s mission to the alliance. In response, Russia suspended its mission.Employees of the Russian Embassy in Prague board a Russian plane with expelled Russian diplomats, May 29, 2021.Gabriel Kuchta/Getty ImagesRussian espionage, with help from locals, has targeted military and space-related technology in Europe. Russian operatives are also suspected of carrying out or attempting sabotage and assassinations inside European countries.In 2014, two warehouses in the Czech Republic belonging to the arms company EMCO were bombed. EMCO had been supplying weapons to Ukraine, and two Russians with the GRU, Russia’s military-intelligence service, were suspected of conducting the attack.A high-ranking GRU officer was in Bulgaria a year later when EMCO’s Bulgarian owner and two others were poisoned. All three survived. The same GRU officer was also behind the attempted assassination of Skripal and his daughter in the UK in 2018.German courts have also found that Russian agents assassinated a Chechen dissident in Berlin in 2019.Advancing Europe’s securityA demonstrator with a sign reading “Russian Embassy out of Germany” at a protest outside Russia’s Embassy in Berlin, April 2, 2022.JOHN MACDOUGALL/AFP via Getty ImagesWhile Europe’s fight against Russian espionage has spread across the continent, that spying is more concentrated in a few areas.Belgium and Austria are considered hubs for Russian spy activity, in part because the number of international organizations in Vienna and Brussels allows Moscow to accredit more operatives as diplomats.Some governments have made the problem worse through lax enforcement or indifference.”The Austrian and Hungarian governments are notorious for their open-door attitude toward Russian agents,” Miller told Insider. Tellingly, Hungary, which maintains good relations with Moscow, has not expelled any Russians during this wave of expulsions.Flags of member countries at NATO headquarters in Brussels, November 26, 2019.ReutersIn response to Russia’s increasing malign activity, NATO created the Joint Intelligence and Security Division in 2016. The JISD was meant to increase the alliance’s intelligence and counter-intelligence capabilities. NATO strengthened the unit in 2020.As a result of the warehouse bombing in the Czech Republic, NATO conducted a “sweeping audit” of the intelligence presence in Russia’s diplomatic outposts in Europe and found them to be “packed” with Russian agents, according to the Economist.Russia has responded to the recent expulsions by sending home diplomats from European countries and from the EU itself. This month, Russia’s deputy foreign minister said Moscow was still assessing the expulsions of its personnel but that “we advocate for diplomatic channels to remain open.”Miller said the expulsions by European countries were a good step, “but after three decades of giving Russian agents free reign across the continent, there’s much more that needs to be done.”Constantine Atlamazoglou works on transatlantic and European security. He holds a master’s degree in security studies and European affairs from the Fletcher School of Law and Diplomacy.