Those who practice criminal law — including the three of us — have long predicted that Paul Manafort, the disgraced former Trump campaign chairman, would eventually fold to Robert Mueller, the special counsel. On Friday, we were proven right: Mr. Manafort and the special counsel have reached an extensive cooperation agreement, among the most significant developments in Mr. Mueller’s investigation to date.

The capitulation came in spectacular fashion: According to prosecutors, Mr. Manafort has already participated in a so-called proffer session, in which he described information that investigators deemed valuable. Mr. Manafort’s agreement will also require him to give further interviews without the presence of his own counsel, turn over documents and testify in other proceedings. His surrender is complete. We will soon see what it means for the president.

Mr. Manafort’s cooperation obviously represents an enormous pivot from the previous strategies he had seemed to be pursuing — trying to persuade a jury to ignore the strong evidence against him based on personal sympathies or, if that failed, to obtain a pardon from President Trump. In our estimation, this about-face represents a realistic judgment on his part, not to mention vindication of Mr. Mueller’s strategy so far.

A quick pardon was always a long shot. It would be hard for Mr. Trump to justify politically, even given his willingness to use the pardon power with seeming abandon. A pardon would not protect Mr. Manafort from the wide array of state charges that could be brought against him.