Ever since Uber showed up in Europe in late 2011, the American ride-booking service has faced vocal opposition.

Some of its drivers have been attacked by angry taxi drivers in Paris. Two of the company’s most senior European executives have stood trial on charges of running an illegal transportation service in France. And taxi associations from London to Frankfurt have accused Uber of flouting local rules and undermining European rivals. The company denies the accusations.

These heated battles will culminate on Tuesday in arguments before the European Court of Justice, the region’s highest court, which will most likely determine how Uber can operate across the European Union, one of the company’s largest international markets.

At stake is the ride-booking service’s often aggressive worldwide expansion. Uber has opened in more than 300 cities on six continents. That has helped the American tech company reach an eye-popping valuation of $68 billion, making it one of the most successful start-ups ever to come out of Silicon Valley.