It took longer than Robert S. Mueller III probably wanted — and longer than Paul Manafort probably should have waited — but Mueller on Friday finally landed his golden goose.

Manafort’s plea deal with prosecutors includes a requirement that he cooperate with Mueller’s Russia investigation. Manafort’s lawyer Kevin Downing was asked if it was a full cooperation agreement, and he responded, “It is.”

This makes him the latest former aide to President Trump to flip. But he’s also likely the biggest, at least when it comes to the Russia probe. Although he was in a position to potentially know a lot, we don’t know what Manafort will flip on — or whether any of it might implicate Trump. But it has been evident from Mueller’s dogged pursuit of him and from Trump’s commentary on the matter that this has been the main event for everyone involved.

While Mueller has farmed out other cases, including Michael Cohen’s, to other prosecutors, he kept Manafort’s for himself. He did so even as Manafort wasn’t charged with crimes specifically pertaining to Mueller’s mandate on Russia’s 2016 interference — the underlying crimes all predate the 2016 campaign — and he did so even as it meant prosecuting two separate trials, one of which concluded with no deal.

As I argued last month, it was readily apparent that Mueller was keeping the Manafort case for himself because of Manafort’s potential to flip. Cohen was seen as a target for flipping, too — and he eventually did — but Mueller even sent that one to the Southern District of New York. The cases of other political consultants who, like Manafort, were facing suspicions of illegal foreign lobbying were also sent up north.

Manafort was special, and his case was evidence that Mueller had hardly thrown in the towel on the collusion question. Here’s what I said in August:

Critics of Mueller’s investigation, including President Trump, have seized upon the Manafort trial as an example of what Mueller hasn’t produced — specifically, anything proving collusion between the Trump campaign and Russia. Manafort is charged with financial and other crimes that predate the 2016 election.

But experts say the fact that Mueller is farming out things like the [Tony] Podesta, [Vin] Weber and [Gregory] Craig cases — and even the Michael Cohen case, which also went to SDNY — reinforces the idea that Manafort is still seen as a potentially key figure in the Russia collusion case. And that goes for basically anything else Mueller has apparently kept under his purview, up to and including the Seychelles meeting, Erik Prince, George Nader and Roger Stone. Mueller has now shown he’ll hand off multiple things that don’t further that specific aim, and everything that hasn’t been handed off would logically still seem potentially pertinent to the collusion probe.

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So why would Mueller refer the Cohen case (along with Podesta-Weber-Craig) to SDNY but not the Manafort one?

The one obvious difference would seem to be that Manafort was a key figure in the Trump campaign and had existing ties to Russian interests that could conceivably be involved in the case’s future. While Cohen may know derogatory things about Trump — and reportedly is even telling people Trump knew about the Trump Tower meeting — Manafort may still be viewed as more of a potential collusion witness.

Through it all, though, it was looking more and more as if Mueller’s bet wouldn’t pay off — that it would all be for naught, possibly because Trump was going to pardon Manafort regardless. Manafort resisted a cooperation deal even as he was being convicted of eight counts at his first trial, in Virginia. And even this week, as news of plea negotiations broke, ABC News reported that Manafort’s lawyers were not entertaining a cooperation agreement as part of the deal.

That wound up being a head-fake. But it made sense, given that cutting a deal after you’ve already been convicted in one case is hardly ideal, and leaves you far less leverage. Why on Earth would you wait that long, after all, if you were just going to flip anyway?

And in that way, Manafort appears to have miscalculated. The new plea deal still requires Manafort to forfeit millions of dollars in assets, and it’s unclear how it will impact his ultimate sentence.

Trump, too, seems to have miscalculated, doing a premature victory dance when Manafort resisted flipping during the first trial. Trump tweeted that prosecutors had “applied tremendous pressure on him and . . . he refused to ‘break’ — make up stories to get a ‘deal.’" The president added: “Such respect for a brave man!”

On Friday, that “brave man” was finally cracked by another brave man who is looking increasingly formidable: Robert S. Mueller III.