Mr. Lewis said the debate will be a test for the relationship between the United States and European Union, which was strained during the Trump administration on issues like digital taxes. A trade group representing big American tech companies, called the Internet Association, has already complained to officials in Washington about the new European rules.
Veterans of past European debates said the challenge will be translating the law’s lofty ambitions into strong enforcement, an area where previous E.U. policies have fallen short.
Europe’s landmark 2018 online privacy law, called the General Data Protection Regulation, has been criticized for not fulfilling its promise because of lack of enforcement. Despite a limited budget, Ireland is responsible for regulating all tech companies with a European headquarters within its borders — including Facebook, Apple and Google — and issued only its first fine of a major tech platform on Tuesday with the penalty against Twitter, more than two years after the law was enacted.
“For the E.U., it is important to get its priorities right in practice and not just talk about them,” said Marietje Schaake, a former member of the European Parliament who now teaches at Stanford University.
The European debate is already turning some companies against one another. On Monday, Facebook issued a statement urging European regulators to act against Apple, part of an ongoing feud between the two companies over Apple’s App Store policies, which Facebook said “harm developers and consumers.”
Raegan MacDonald, head of public policy in Brussels for the Mozilla Foundation, which operates the Firefox browser, called the efforts in Europe a “once-in-a-generation” opportunity, particularly the transparency rules that would provide important insights about how the companies operate.
“What this is really about at its core is how people experience the web — the misinformation in our feeds, the recommendations that are being pushed toward us, or the creepy ads we’re seeing and don’t know why,” she said. “If this is done well, this could be game changing regulation for platform accountability.”