Google has illegally built “monopoly power” with its web advertising business, a federal judge in Virginia has ruled, siding with the Justice Department in a landmark case against the tech giant that could reshape the basic economics of running a modern website.

A U.S. judge's ruling that Google has illegal monopolies in ad technology sets up the possibility of U.S. prosecutors seeking a breakup. Here's what the case involves and what Google owner Alphabet faces from here.

It was the second time in a year that a U.S. court found that the company had acted illegally to remain dominant.