Last summer, two weeks after Donald Trump won the Republican nomination, his son met a Russian lawyer. The meeting, revealed by the New York Times, took place at Trump Tower in Manhattan. Those present were Donald Trump Jr, Donald Trump’s son-in-law Jared Kushner and the candidate’s campaign chairman, Paul Manafort.

The lawyer was Natalia Veselnitskaya. She had previously lobbied against a US law that sanctioned prominent Russians for human rights abuses. The meeting came about via a backchannel. A Russian-Azeri businessman, Aras Agalarov, and his son, Emin, who hosted Trump for the 2013 Miss Universe pageant, got in touch with Rob Goldstone, a British publicist and former tabloid reporter, who in turn contacted the Trump campaign.

On Tuesday Donald Trump Jr published an exchange of emails with Goldstone. They are damning. On 3 June 2016, Goldstone wrote that “Emin just called and asked me to contact you with something very interesting.” He said that Aras had met with Russia’s crown prosecutor and offered to provide “some official documents and information that would incriminate Hillary and her dealings with Russia and would be very useful to your father.”

Goldstone added: “This is obviously very high level and sensitive information but is part of Russia and its government’s support for Mr Trump - helped along by Aras and Emin.” Trump Jr seemed receptive. Seventeen minutes later he emailed back to say he appreciated the offer, adding: “If it’s what you say I love it especially later in the summer”.

Further emails from Goldstone fixed the meeting with Veselnitskaya for 9 June. He described her as a “Russian government attorney flying over from Moscow”.

How serious is all this?

Very. This looks like the first concrete proof of collusion. FBI special prosecutor Bob Mueller is currently investigating if the Trump campaign conspired with Russia during the 2016 US election. In late 2015 the British spy agency GCHQ noticed suspicious contacts between Trump associates and suspected Russian agents. These meetings took place in London and in Europe and continued in 2016. Meanwhile, in the US senior Trump figures saw or spoke with the Russian ambassador, Sergei Kislyak.

Trump has repeatedly insisted none of this amounts to collusion. But the details of the Trump Jr meeting and the Goldstone emails are excruciating for the White House: Trump Jr was explicitly told in the email that the lawyer would bring him “sensitive” and compromising material on Clinton. This may not have happened. But Trump Jr shows intent. He could have refused to meet. Or reported the approach to the FBI. He did neither. The fact that the meeting went ahead suggests that people in the Trump campaign believed the Kremlin held dirt on Clinton.

What about timing?

The timing of the meeting – 9 June 2016 – is critical. It took place after two Russian spy agencies had hacked the Democratic National Committee but before this was made public. Later that day Trump attacked Clinton over her emails, tweeting: “Where are your 33,000 emails that you deleted?”

In July, WikiLeaks released 20,000 DNC emails on the eve of the Democratic National Convention. On 22 July, Trump told a press conference in Florida: “Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing.”

What does this mean legally?

Historically, presidents aren’t prosecuted in court, they’re prosecuted (if at all) in Congress, which Republicans currently control. If Democrats gain control of Congress, or the evidence against Trump builds and Republicans turn on him, the president may be charged with violating his oath of office, abuse of power or obstruction of justice.

Meanwhile, Trump Jr or others may be tripped up, by campaign law restrictions that preclude accepting assistance from foreigners. Normally, that would mean cash. But it might also include favours in kind, such as damaging information on a rival. There is the wider question of conspiracy. The Russian hackers who stole Democratic emails committed crimes. Anyone who dealt with them, and tried to secure benefit from these crimes, could be roped into a count of conspiracy. However, as president, Trump has the authority to pardon any federal crime.

What did the Kremlin want from the meeting?

In 2012 the Obama administration – with bipartisan support – passed the groundbreaking “Magnitsky act”. It sanctioned a group of corrupt Russian officials who were implicated in the death of Sergei Magnitsky, a Russian lawyer who died in prison in 2009. The act infuriated Vladimir Putin who responded by banning the adoption of Russian children by US couples.

Q&A What is the Magnitsky Act? Show Hide Sergei Magnitsky was a Russian lawyer who investigated a massive $230m tax fraud in 2007.

After he revealed the scam, Magnitsky was arrested by the same officials whom he had accused of covering up the racket and was imprisoned. He died in jail after being denied medical treatment. Russia accused him of committing the fraud himself and even put him on trial posthumously.

After a long campaign by his associates, the Magnitsky Act was passed by Congress in 2012, banning entry to the US and freezing assets of officials believed to have been involved in Magnitsky's persecution and death. Russia responded by banning a list of US citizens it deemed hostile to Russia, and blocking the adoption of Russian children by US citizens – a controversial move that led critics to suggest the Kremlin was punishing Russia’s most vulnerable children.

In 2014, the US and EU imposed further sanctions on Russia after Putin’s covert invasion of eastern Ukraine and his annexation of Crimea. As a result, many of Putin’s billionaire friends are unable to travel to the west. Nor can they hold US bank accounts. The Kremlin’s top foreign policy priority is to have the sanctions dumped. It has expended considerable effort on soft power lobbying. Veselnitskaya’s meeting with Trump Jr was part of this operation.

How much does the FBI know?

The Kremlin says it has no connection with Veselnitskaya. This claim has to be treated sceptically. Moscow typically subcontracts “influence operations” to oligarchs and third parties for reasons of deniability. It used WikiLeaks to distribute its DNC hack and in this case appears to have employed the Agalarovs. As a high-profile Kremlin lobbyist Veselnitskaya is likely to have been under US surveillance. The US had previously denied her a visa. Certainly, the FBI got hold of the email chain before Trump Jr published it. Three people who read it briefed the New York Times this week. Trump has accused GCHQ of spying on him at the behest of the Obama administration. The claim is untrue. The reason Trump’s associates may have been bugged is because they were meeting with known or suspected Russian intelligence operatives. The Russians were the targets.

What don’t we know?

Trump Jr has given different accounts of the meeting. At first he said its subject was the adoption of Russian children. Then, when confronted with more evidence, he admitted that he had been promised compromising information on Clinton. Now he had released the evidence. What exactly the Kremlin wanted from the meeting is unclear. The overall goal, we now know from the emails, was to help Trump. By this point, it was sitting on a heap of Democratic party emails that had been hacked or not released. We don’t know precisely what Veselnitskaya said. Nor do we know how Trump Jr and Manafort responded.

What other Russian connections are there?

Lots. Before joining Trump’s campaign Manafort spent over a decade working in the former Soviet Union. His previous client was Viktor Yanukovych, the ex-president of Ukraine who fled to Russia in 2014. The FBI is investigating Manafort’s dealings with Russian oligarchs and his unusual cash purchases of New York real estate. He denies wrongdoing.

Meanwhile, Kushner met several prominent Russians in 2016 including Kislyak and Sergei Gorkov, a Russian banking executive and graduate of the FSB spy agency’s academy.

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