It wouldn't be a surprise if Russia had compromising information on Donald Trump. He first came to the Soviet Union in 1987 to talk about building hotels. And he was willing to be exposed.

World leaders have their bodyguards and own intelligence, both of which he lacked. His bodyguards wouldn't have been able to operate in the country, so he would have had to rely on Russians. He may have thought he was never going to be a politician, so didn't need to bother about being careful.

But the President-elect is not stupid. Surely someone told him about the KGB? If he went to have meetings with Politburo, he would have been screened by KGB members. This is the tradition of the Soviet system. They have always gathered compromising information - kompromat - on anybody of interest to manipulate them, whether allies or not.

The gathering of kompromat is the hallmark of Russian intelligence. Opposition politicians have repeatedly been tape-recorded. On several occasions, actors would play those being targeted.

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The event doesn't need to be real, so long as the film can be leaked to discredit them.

A good example of this happened in 1999, after Russia's chief prosecutor, Yuri Skuratov, threatened to go after members of Boris Yeltsin's family.

Photographs surfaced of him fooling around in bed with two prostitutes. He was fired after the director of the FSB vouched for their authenticity.

Who was head of the security service at the time? Vladimir Putin. What was the practice in 1999 is still alive and well.

The Russians have a variety of ways of obtaining information. If they want to target someone staying at a certain hotel, there are ways they gain access to plant whatever devices they need.

Fire inspectors could come by and tell the owner that their hotel is violating safety regulations, and would need millions spent to avoid being shut down. There would be one solution of course, which is to let the intelligence agencies have access to the rooms where certain individuals are staying. It'd not be wise for a hotelier to resist the Russian state.

The Russians don't just know how to gather kompromat, but how to use it to influence their target. A middle-man could be asked to innocently allude to what was on the film. "By the way, Tania and Natasha send their best" perhaps. Or they may get a private email inviting them to download some choice moments.

Critics argue that Trump's warmth about Putin is a sign that he is in Russia's pocket. But he isn't defending Russia; he is defending his legitimacy.

President Trump will still be useful to Putin, but just for the first six months while the Russians try to work him out.

Putin's favourite weapon against Western politicians is unpredictability. But he loses that against Trump, as he has to deal with another bully like him.

Trump was a private individual when he visited Russia, but now nears presidential office. He may not worry about what kompromat Russia has on him, but he'll know everything now is in the public eye.

Dr Igor Sutyagin is senior fellow in Russian Studies at the Royal United Services Institute.