Sheryl Sandberg was seething.

Inside Facebook’s Menlo Park, Calif., headquarters, top executives gathered in the glass-walled conference room of its founder, Mark Zuckerberg. It was September 2017, more than a year after Facebook engineers discovered suspicious Russia-linked activity on its site, an early warning of the Kremlin campaign to disrupt the 2016 American election. Congressional and federal investigators were closing in on evidence that would implicate the company.

But it wasn’t the looming disaster at Facebook that angered Ms. Sandberg. It was the social network’s security chief, Alex Stamos, who had informed company board members the day before that Facebook had yet to contain the Russian infestation. Mr. Stamos’s briefing had prompted a humiliating boardroom interrogation of Ms. Sandberg, Facebook’s chief operating officer, and her billionaire boss. She appeared to regard the admission as a betrayal.

“You threw us under the bus!” she yelled at Mr. Stamos, according to people who were present.

The clash that day would set off a reckoning — for Mr. Zuckerberg, for Ms. Sandberg and for the business they had built together. In just over a decade, Facebook has connected more than 2.2 billion people, a global nation unto itself that reshaped political campaigns, the advertising business and daily life around the world. Along the way, Facebook accumulated one of the largest-ever repositories of personal data, a treasure trove of photos, messages and likes that propelled the company into the Fortune 500.