The decades-long spy conflict between Russia and the West is intensifying over the Ukraine war. But what are Russia's intelligence services suspected of doing and how will their officials' expulsion from capitals affect Putin's clandestine overseas operations?
When Russia first targeted its military forces on Ukraine in 2014 it also unleashed its intelligence services on the West - from interfering with the US elections using cyber attacks to poisonings and sabotage in Europe.
But in recent months the spy war has intensified as Western countries have sought to hit back and inflict lasting damage on the ability of Russian intelligence to carry out covert operations. This is symbolised by the unprecedented expulsion of 500 Russian officials from Western capitals.
Formally, these officials are described as diplomats, but the majority are believed to be undercover intelligence officers. Some will have been carrying out traditional espionage - cultivating contacts and recruiting agents who can pass on secrets - something Western countries do inside Russia too.
But some were believed to be carrying out what Russians call "active measures". These range from spreading propaganda, to more aggressive covert activity. Poland said the 45 Russians it expelled were involved in actions to "undermine the stability" of the country.
Since 2014, Western intelligence agencies have been working to identify Russian spies involved in such activities. One of those is GRU Unit 29155 of Russian military intelligence, which is believed to be tasked with sabotage, subversion and assassination.
It took nearly seven years to find out the unit were behind a huge explosion that tore apart an ammunition depot in a Czech forest in October 2014. They included some of those later involved in the UK's Salisbury poisonings of 2018.
The same team also tried to poison an arms dealer in Bulgaria who had stored weapons in the Czech depot - one theory was that the blast and poisoning was linked to his supply of weapons for Ukraine where the conflict had just begun.
Members of that unit were also involved in getting pro-Russian leaders out of Ukraine in 2014. It remains closely watched by Western intelligence.
But man-marking individual spies is expensive work. While Western spies in Russia have long been subject to round-the-clock surveillance, their Russian counterparts in Western capitals have not.
"The larger the presence is, the more difficult it is to keep a lid on exactly what they are up to," one US official told the BBC.
But this may now be changing. Western officials say the recent expulsions are more than a symbolic gesture of protest but part of the wider push to degrade Russia's capacity to do harm. Some spy-catchers also say the mass expulsion is long overdue. The Russians have been laughing at us for our tolerance of their presence, says one official.
"We are trying to inflict a cost on Russia to reduce its offensive capabilities and its ability to project threat against its neighbours and the West," one official says. "A number of European nations have taken action to reduce the Russian intelligence service capability across Europe. All of these are steps designed to reduce its threats to us."
Some countries are believed to have had a particularly significant presence. Berlin expelled 40 Russians. However, a Western intelligence official said they believed Germany had previously housed closer to 100 Russian intelligence officers, acting like an "aircraft carrier" for their operations.
Why has the UK not expelled anyone? Officials say all of them were kicked out after Salisbury and the only spies left are "declared" officers who act as liaisons for formal contacts. They are likely to be watched by MI5 for any sign they are carrying out any covert acts on the side.
In the US, expulsions are based on investigations into each individual. "All the determinations on who to expel are based on intelligence collected by the FBI based on what they are doing," explains a US official. Western countries have been co-operating to ensure anyone expelled cannot simply apply for a visa in another country.
Security officials say they believe the volume of expulsions over a short period will have a "debilitating" impact on Russian intelligence as it scrambles to work out how operations can be continued and who can be placed where.
Russia has retaliated by expelling Western diplomats. In practice, more of these are likely to be "real" diplomats rather than spies. One of the complaints from Western security services has long been the imbalance in the number of Russian diplomats in Western countries, and the proportion who are spies compared to those from the West serving in Moscow. Russia expelled 40 Germans but that makes up around a third of the entire diplomatic presence in its capital.
The invasion of Ukraine may offer other opportunities. Past events like the crushing of the Prague Spring by Moscow in 1968 caused disillusionment among some within the secret state in Moscow, opening the way for their recruitment as Western agents.
In Washington DC, the FBI has targeted online advertisements to people in close proximity to the Russian Embassy, according to a Washington Post report. They encouraged them to talk to the FBI, using footage of Vladimir Putin publicly embarrassing the head of Russia's foreign intelligence agency, the SVR.
Since 2014, Ukraine has also been the epicentre for a more brutal covert struggle, with each side trying to recruit and root-out spies but also with assassinations of high-ranking Ukrainian officials.
Western intelligence agencies and special forces have also been training Ukrainian counterparts for years, alongside more overt military assistance. They have helped catch Russian spies and offered training in covert action, including by the CIA's Ground Branch.
The spy battles could still escalate, particularly as covert activity presents one option for Moscow to target supply lines bringing in military aid for Ukraine. A missile strike on convoys or facilities in Poland would be highly risky as it could trigger NATO's Article 5 self-defence principle leading to all-out conflict.
But Western intelligence officials say they have concerns that the kind of sabotage operation seen in the Czech Republic in 2014 could be attempted in Poland given its key role as a staging post for supplies going into Ukraine.
These type of clandestine operations are often carried out by Russians who travel in and out of a country rather than diplomats. But embassies provide the enabling infrastructure for their activities to take place, one Western intelligence official explains.
And the hope will be that the large-scale expulsions will make that, as well as traditional spying, much harder now, not least because there will be fewer spies to keep tabs on.