Silicon Valley’s tech giants have long regarded antitrust scrutiny as an irritating cost of doing business. There will be investigations, filings, depositions and even lawsuits.

Yet courts move slowly, while technology rushes ahead. Time works to the companies’ advantage, as the political winds shift and presidential administrations change. That dynamic often opens the door to light-touch settlements.

But the stakes rose sharply for Google on Thursday, when a federal judge ruled that the company had acted illegally to build a monopoly in some of its online advertising technology. In August, another federal judge found that Google had engaged in anticompetitive behavior to protect its monopoly in online search.

Antitrust experts said two big antitrust wins for the government against a single company in such a short time appeared to have no precedent.

“Two courts have reached similar conclusions in product markets that go to the heart of Google’s business,” said William Kovacic, a law professor at George Washington University and former chairman of the Federal Trade Commission. “That has to be seen as a real threat.”

The Google decisions are part of a wave of current antitrust cases challenging the power of the biggest tech companies. This week, the trial began in a suit by the F.T.C. claiming that Meta, formerly Facebook, cemented an illegal monopoly in social media through its acquisitions of Instagram and WhatsApp.