More than a year ago, America's antitrust police appeared to have finally gained the upper hand in their long-running campaign to rein in the nation's tech giants. Within nine months, the Justice Department and a group of states had won back-to-back lawsuits against Google over allegations the company created illegal monopolies in internet search and online advertising technology. And a Federal Trade Commission suit accusing Meta of illegally thwarting nascent competition when it bought Instagram and Whats App years earlier had just gone to trial. In all three cases, the threat of harsh, business-wrenching penalties loomed. That included court-ordered breakups.

But on Wednesday, a federal judge decided against a breakup of Google's ad tech business, leaving the company largely unscathed. Another judge ruled similarly in the search case a year ago, citing the fast-moving nature of technology. And in November, a federal judge ruled in favor of Meta, deciding that time and technology had passed the case by. The decisions show the wariness of courts to demand major business changes, even after finding clear-cut antitrust violations, as technology races ahead.

They also reflect how deciding whether violations have occurred is an examination of the past, while determining what to do about them requires a look into the future. Exhibit A? The rapid ascent and disruptive impact of artificial intelligence. "Dynamic markets pose a real challenge for the courts and have been seen as reason for caution," said William Kovacic, a law professor at George Washington University and a former chair of the F.T.C.

The Justice Department filed its search suit against Google in 2020, during the waning months of the first Trump administration. And the Biden administration took an aggressive approach to antitrust enforcement, filing suits that seek to curb the market power of the tech giants. Two more major court tests lie ahead. The F.T.C. and a group of states have sued Amazon, claiming it squeezed sellers in its online marketplace and favored its own offerings. (The F.T.C. sued Amazon again on Monday, claiming the e-commerce giant secretly made advertising more expensive on its website for more than a million companies.) And the Justice Department and states have accused Apple of using its dominance in the smartphone market to thwart competition.