Paul Manafort didn’t just plead guilty to fraud charges Friday, he's now cooperating with the special counsel's Russia investigation. And man oh man, is he ever a star witness based on what he was privy to during the critical five months he managed the campaign of Donald J. Trump and even in the months thereafter. Remember, even though the fraud charges he faced have little to do with Trump directly, his cooperation means he can be called to offer information on anything and everything special counsel Robert Mueller is interested in.

1. The mysterious pro-Russian GOP platform change

Manafort was the de facto campaign manager in July 2016 when Trump captured the GOP nomination and a mysterious change was made to the Republican party platform that weakened U.S. support for Ukraine's efforts to combat a pro-Russian separatist movement in the country.

No one has ever really gotten to the bottom of how or why that change was made, but an RNC delegate has accused Trump of directly ordering it. Manafort may well have more information into how that pro-Russian change was engineered.

2. The Trump Tower meeting

Manafort attended (and took notes!) on the now-infamous June 2016 Trump Tower meeting between top Trump campaign chiefs—including himself, Jared Kushner and Don Jr.—and a Kremlin-linked lawyer (who later admitted to being an agent of the Russian government) and her entourage.

Manafort's involvement in that meeting will be key to the fate of Don Jr. and possibly Trump himself, since he may very well know if Trump knew about the meeting in advance of it. In fact, two days before that June 9 meeting, Trump also publicly promised to give a a "major speech" on "all the things that have taken place with the Clintons" that ultimately never materialized. Just guessing that was a topic of conversation among those heading the campaign, chief among them, Manafort.

3. Trump's solicitation of Russian help in the election

Manafort was also the de facto campaign manager when in July 2016 Trump enlisted Russia's help in the winning the election with the very public appeal: "Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing."

4. Russian interactions with George Papadopoulos

Manafort officially joined the Trump campaign on March 29. Two days later, Trump convened a meeting with his national security advisers, including Papadopoulos, that he later memorialized with a tweeted picture of the attendees. In the meeting, Papadopoulos introduced the idea of arranging a meeting between Trump and Russian President Vladimir Putin. While there's been conflicting reports about how that prospect was received by Trump and key campaign surrogate Jeff Sessions, it clearly kicked off a long dance between the Kremlin and Papadopoulos, who also learned later that Russia had "dirt" on Hillary Clinton. That back-and-forth, meticulously documented in the special counsel's statement of offense for Papadopoulos, took place between late March and mid-August, the time period during which Manafort consolidated power within the campaign and his influence peaked then ebbed (Manafort officially resigned due to his pro-Russian ties on August 19, 2016).