Google on Tuesday begins a marathon battle in a federal court in Washington as it fends off accusations from the U.S. government that it acted unlawfully to become the world's dominant search engine, in the biggest antitrust trial America has seen in a quarter of a century.

Over the course of 10 weeks of testimony involving more than 100 witnesses, Google will try to persuade Judge Amit P. Mehta that the landmark case brought by the Department of Justice is without merit.

Held in a Washington courtroom, the trial is the biggest U.S. antitrust case against a big tech company since the same department took on Microsoft more than two decades ago over the dominance of its Windows operating system.

The Google case centers on the government's contention that the tech titan unfairly forged its domination of online search by entering into exclusivity contracts with device makers, mobile operators, and other companies that left rivals no chance to compete.

Through these payments of billions of dollars every year to Apple and others, Google secured its search engine default status on phones and web browsers, allegedly burying upstarts before they had a chance to grow.

That dominance has made Google parent Alphabet one of the richest companies on Earth, with search ads generating nearly 60% of the company's revenue, dwarfing income from other activities such as YouTube or Android phones.

"Two decades ago, Google became the darling of Silicon Valley as a scrappy start-up with an innovative way to search the emerging internet," the Justice Department said in its lawsuit. "That Google is long gone."

Dozens of U.S. states, led by Colorado, have also joined the battle even though some of their arguments that Google illegally down-ranked sites such as Yelp and Expedia were tossed out pre-trial by Judge Mehta.

90% share

The biggest alleged victims in the case are rival search engines that have yet to eke out a meaningful market share against Google, like Microsoft's Bing and DuckDuckGo.

Google remains the world's go-to search engine, capturing 90% of the market in the United States and across the globe, much of which comes through mobile usage on iPhones and phones running on Google-owned Android.

The company will contend that its success is due to the unbeatable quality of its search engine which has been judged a cut above the rest since its launch in 1998 by founders Sergey Brin and Larry Page.

"In sum, people don't use Google because they have to – they use it because they want to," said Kent Walker, Google president of global affairs in a blog post.

Judge Mehta's ruling is expected many months after the roughly three months of expected hearings.

He could dismiss the case or order drastic remedial action such as a break up of Google's businesses or a revamp of the way it operates.

Whatever the outcome, the ruling will almost certainly be appealed by either side, potentially dragging the case on for years.

Launched in 1998, Washington's case against Microsoft ended in a settlement in 2001 after an appeal reversed an order that the company be split up.

The U.S. government launched its case against Google during the Trump administration and the suit carried over in the transition to President Joe Biden.

Biden has also made a point of challenging tech giants and nominated well-known tech critics to key posts, but with little yet to show for it.

In January, Biden's Department of Justice launched a separate case against Google involving its advertising business and this could go to trial next year.

The company also faces various lawsuits from U.S. states that accuse it of abusing monopolies in ad tech and blocking competition in its Google Play app store.