STORY: Legal teams have gathered in Virginia as Google goes on trial.
The tech titan's latest antitrust showdown concerns allegations it sought to dominate all sides of online advertising technology.
A Justice Department spokesperson said the firm used classic monopoly-building tactics, including eliminating competitors through acquisitions and locking customers into using its products.
She said Google wasn't on trial because it's big - but because it used its size to crush rivals.
The court in Alexandria will hear the case without a jury.
Google's outside attorney Karen Dunn said the case was based on "ancient history", dating back to when the company was still trying to make its tools work with other firms' products.
She said that was no longer the situation, with Google now facing vigorous competition from rivals like Amazon and Comcast.
The case, she said, is "like a time capsule that if you broke it open you would find a BlackBerry, an iPod and a Blockbuster Video card".
Dunn left court after her opening remarks, racing off to another job helping to prepare Vice President Kamala Harris for her debate with Republican presidential candidate Donald Trump.
If Google loses the case, it could be ordered to sell off some ad tech systems.
Stock analysts at Wedbush estimate such products earned Google $20 billion in 2020, or over 10% of its revenue.
More recent figures were redacted in court documents.
Last month, the Justice Department won a separate case against Google over its dominance in online search.
Apple, Amazon and Facebook-parent Meta also all face probes by Washington watchdogs.