Dec. 1 was easily the biggest day so far in the investigation into the Trump campaign’s ties with Russia: In agreeing to a plea deal with Mike Flynn, special counsel Robert Mueller has officially flipped one of Trump’s most senior and earliest backers.

Here’s everything you need to know about the former national security advisor’s guilty plea, Flynn’s promise of cooperation with the FBI investigation, and what it could all mean.

What has Flynn pled guilty to?

He’s admitted to telling four lies to FBI agents when they interviewed him on Jan. 24 2017. The four lies:

That on a Dec. 29, 2016 phone call, he didn’t ask Russian ambassador Sergey Kislyak to not respond to the Obama administration’s new sanctions on Moscow by escalating the situation. (Flynn did request this.) That he didn’t remember Kislyak later telling him that the Kremlin decided to “moderate its response” according to this request. (He presumably did remember.) That on a Dec. 22 2016 phone call he didn’t ask Kislyak to delay or help defeat a UN Security Council resolution condemning Israel’s settlements in Palestine. (He did.) That Kislyak never described to him Russia’s response to that request. (Kislyak presumably did. Russia ended up voting for the resolution and the Obama administration abstained, meaning the resolution passed.)

Lying to the FBI carries a maximum sentence of 5 years in jail and a $250,000 fine.

That’s all?

Yes—and that’s an important point. Flynn could have been in line for much heavier charges.

On the above evidence alone, Mueller might have nailed Flynn on the Logan Act, which bans private citizens from conducting unauthorized diplomacy with foreign governments (the act has never actually been enforced). In this case, Flynn was directly briefing against US government policy.

Potentially far more damaging is the probe into Flynn’s alleged role in a purported plot to kidnap dissident Turkish cleric Fethullah Gülen from the US. He may also have violated (paywall) the Foreign Agents Registration Act (FARA) by not declaring his work for a Turkish government-linked company until March this year (far later than the 10-day deadline). People are rarely prosecuted for FARA, but Mueller has already charged former Trump campaign chair Paul Manafort and his aide Rick Gates under the act. It also carries a five-year sentence.

Finally, as a retired general, Flynn may have violated the emoluments clause by taking payments from foreign governments (Russia and Turkey) without permission. That’s not a criminal offense, but he’d have to return the cash.

Why are they letting him off so lightly?

Mueller threw the kitchen sink at Manafort and Gates, with 12 criminal charges against them, but Flynn has just one. That’s because Mueller wants to bludgeon Manafort and Gates into cooperating, while Flynn has already been doing so.

To get this kind of light deal, Flynn would probably have had to offer highly damaging information about other people in last year’s presidential campaign, writes University of New Hampshire law professor and former public defender Seth Abramson (Abramson’s Twitter account is a must-read for those following the Russia probe and heavily informs this piece). What’s more, that information has to be on someone higher “up the food chain” in the organization.

For someone of Flynn’s seniority, there are very few people who fit that bill. President Trump and vice-president Mike Pence are two of them. Whether Trump’s son-in-law and right hand man Jared Kushner is a big enough fish could be open to interpretation. Abramson argues that Flynn could only have got this very easy deal by offering incriminating information on Trump or Pence.

The first early sign that he’s done just that is ABC’s report that Flynn will testify that Trump ”directed him to make contact with the Russians” during the transition.

Update, Dec 3: ABC initially reported that Trump had made this order during the campaign. It has now corrected that report to reflect that he did so after he was elected.

Bloomberg has further reported that Kushner ordered him to contact foreign ambassadors, including the Russians, over the Israel resolution.

What else is at stake over Flynn’s testimony?

First in the spotlight will be Kushner. With Flynn cooperating, Mueller almost certainly knew about Kushner’s reported role in lobbying against the Israel resolution when he called Kushner in for questioning last month. If Kushner lied, that could see him liable to the same charges that took down Flynn. There will also be questions over whether Kushner’s alleged actions fall foul of the Logan Act.

Mueller will also want to know exactly what Trump said when allegedly ordering Flynn to request a moderate response from the Russians about sanctions. As former US attorney Preet Bharara points out, “Trump is not known for caution or modest requests.” Mueller can’t legally indict Trump, since he’s the president, but anything he finds out will inform members of Congress who wish to impeach him.

Similarly, Mueller will have probed exactly what Mike Pence knew about this all. Flynn was cast (paywall) as the culprit—and eventually fired—when Pence said on television in February that the former general had not discussed sanctions on the phone with Kislyak. If Pence knew the truth, he would face serious political repercussions for lying to the public about a national security matter.

Finally, don’t forget Manafort and Gates. Those two have been stewing under house arrest for a month, presumably weighing whether to testify against the president in exchange for a plea deal. Seeing Flynn flip is a serious test of their resolve.

How about all the collusion stuff?

Indeed, everything we’ve covered so far came after the election. We’ve not even touched on the suggestions that the Trump campaign actively colluded with the Kremlin to defeat Clinton. As one of the campaign’s very top officials, Flynn will have an extraordinary amount more insight than George Papadopoulos, a junior foreign policy adviser and the only previously confirmed collaborator for Mueller.

The FBI will be demanding everything Flynn can give them. Everything he knows about every single contact with Russia during the campaign. All the details he has on the allegations in the infamous Steele dossier. Exactly why Trump’s team removed (paywall) anti-Russia language on Ukraine from the Republican Convention in 2016.

Flynn was one of the most trusted members of Trump’s team. The president has been in contact with him as recently as April, and even after firing him called him a “very good man.” It’s likely that no one outside Trump’s family knows more about his campaign and its ties to Russia than Michael Flynn.

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