As Silicon Valley companies become overwhelmingly dominant, the vast majority of efforts to check their power have come not from U.S. lawmakers or consumers, but from European regulators. A year ago, the European Commission slapped Google with a record-breaking $2.7 billion fine for prioritizing its own shopping service over competitors in search results. And on Wednesday, Europe levied an even steeper fine against the Mountain View, California-based tech giant, hitting Google with a $5 billion fine over an anti-trust case involving its popular Android operating system. In its decision, the Commission ruled that the terms Google set for device-makers who sold phones running the Android operating system—which powers roughly 80 percent of smart mobile devices in Europe—were harmful for competition, and ultimately illegal.

The decision stems from a complaint brought by Google’s European and U.S. rivals in 2015—Google, they pointed out, had required some smartphone-makers to install Google apps, like its Chrome browser and Google Search, on phones that used Android. If device-makers didn’t do this, Google wouldn’t let them install the Google Play Store, where Android users buy and install apps and games for their phones. Not only did Google break the law by requiring device manufacturers to install its programs as a condition for licensing its app store, but its actions allowed it to use Android to dominate search and advertising—Google’s primary source of revenue. “Our case is about three types of restrictions that Google has imposed on Android device manufacturers and network operators to ensure that traffic on Android devices goes to the Google search engine,” European Commissioner for Competition Margrethe Vestager said in a statement. “In this way, Google has used Android as a vehicle to cement the dominance of its search engine. These practices have denied rivals the chance to innovate and compete on the merits. They have denied European consumers the benefits of effective competition in the important mobile sphere. This is illegal under E.U. anti-trust rules.”

If Google doesn’t change its practices in 90 days, it could face additional fines. The company intends to appeal the ruling, according to a blog post C.E.O. Sundar Pichai published Wednesday. “Android has created more choice for everyone, not less,” a Google spokesperson told the Hive. “A vibrant ecosystem, rapid innovation, and lower prices are classic hallmarks of robust competition.”

As the Big Five tech companies—Facebook, Alphabet, Microsoft, Amazon, and Apple—continue to pursue bigger deals in search of market supremacy, their leaders remain relatively unaccustomed to oversight. As Luther Lowe, Yelp’s public-policy chief and a frequent critic of Google’s overreach, explained to me on Wednesday, the company has long relied on the dominance of its smartphone operating system to protect its search supremacy. “In the U.S. and Europe, Google has a staggering 98 percent market share of search on smartphones, including iPhone, where half of all searches have local intent. This allows them to put their thumb on the scale. For example, when a mother searches for a pediatrician in Berlin, instead of being matched with the most relevant information on the Web, such as from native local-review service Jameda.de, Google redirects that user into their in-house product—even though they concede it has less content and a lower ranking.”

With the E.U. clawing back against Silicon Valley in a material way, however, that sort of dominance could ultimately be undone. Europe has certainly been more adversarial toward tech companies than the United States—just months ago, it enacted G.D.P.R., an ultra-strict digital-privacy law, with which myriad tech and media companies scrambled to comply. And toward the end of last year, Apple was forced to defend its decision to move billions of dollars in offshore subsidiaries to the island of Jersey, which is outside of the E.U.’s jurisdiction, after regulators cracked down on its Irish tax haven. If tech companies, already struggling to remain in conservatives’ good graces, tilt the scale too far, U.S. lawmakers may find a ready model to cut them down to size.