For the past six weeks, the Justice Department and dozens of top state prosecutors have tried to prove that Google illegally used its monopoly power to ensure its search engine remained on top.

Now, it's Google's turn to lay out for the judge in this landmark trial why it thinks the government is wrong.

The $1.7 trillion company is expected to argue that it dominates in search because it has the best technology and, so, people prefer it. On Monday, it's bringing in a star witness to testify: Alphabet and Google CEO Sundar Pichai.

Pichai has a deep history with Google's search engine business. He's expected to testify that Google has worked to achieve the best search product for consumers, and that has only helped to serve competition.

The court battle between Google and the government is the first major tech monopoly case to make it to trial in decades and the first in the age of the modern internet. The last time a case of this magnitude went to court was in 1998 against Microsoft. In that case, the judge ruled in favor of the government, saying Microsoft violated antitrust laws.

"The Microsoft case was, at the time, talked about as the case of the century," said John Kwoka, a Northeastern University economics professor who researches antitrust. "But that was the last century, so we have a new one: new century, new potential landmark case."

Though the topic of antitrust may sound bland, the Google trial has experienced its fair share of intrigue. There's been allegations of document destruction and details about billion dollar deals between the world's richest companies. The government brought around 30 witnesses to testify – including experts, psychologists and top executives from Apple and Microsoft – aiming to prove Google broke the law.

"Google illegally maintained a monopoly for more than a decade," Kenneth Dintzer, the Justice Department's lead lawyer, said in opening statements on Sept. 12. "If Google sets the rules, it will always be to their advantage."

The government's case

To understand how Google will counter the government, it's necessary to know what the Justice Department alleges.

The government bears the burden of proof to show Google harmed competition. Its case centers on claims that Google illegally orchestrated its business dealings to ensure that it's the first search engine people see when they turn on their phones or computers.

The way Google has done that is through exclusive agreements with device makers, web browsers and mobile carriers, like Apple, Mozilla and AT&T, the Justice Department says. The end result is that Google has become so massive it's allegedly impossible for rivals to compete – and ultimately consumers are left with no choice other than Google.

During the trial, the Justice Department presented evidence that Google paid Apple at least $10 billion a year to guarantee it was the default search engine on devices like the iPhone and iPad. Eddy Cue, Apple's senior vice president of services, testified that those deals were mutually beneficial to both companies.

The Justice Department called dozens of other witnesses. They included Microsoft CEO Satya Nadella, who testified that he unsuccessfully tried for years to get Apple to switch the default browser on its devices from Google to Microsoft's Bing. Without being able to do that, he said, even a company as big as Microsoft couldn't compete.

"Everybody talks about the open web, but there is really the Google web," Nadella said during his testimony. "The distribution advantage Google has today doesn't go away."

Executives from other smaller search engines, like DuckDuckGo and Neeva also testified that Google's exclusive deals effectively quashed their potential to gain market share.

While some information about Google's business dealings came out in court, a lot was presented behind closed doors. Cue testified for four hours, but more than half of that was closed to the public. Throughout the course of the trail, Google continually fought to seal documents and shutter proceedings in public court.

It became so pervasive that The New York Times and other major news organizations filed a court motion imploring the judge to ensure the case was conducted in an open courtroom.

Google's play and its star witness

Throughout the course of the trial Google has brushed aside claims that its exclusive agreements with device manufacturers are what gives its business a leg up. Rather, the company claims, it's the quality of its products. Search engines like Bing just don't measure up, said Google's lead lawyer John Schmidtlein during opening statements.

"Microsoft has failed to invest, failed to innovate in a manner comparable to Google, in many areas that have nothing to do with scale," Schmidtlein said.

Google began its defense last Thursday and is expected to continue to hit on this same idea over the next three weeks. And there are few people more qualified to talk about its search products than CEO Sundar Pichai.

When he first joined the company in 2004, his job involved working on the Google search toolbar. He later led the team that built the company's Chrome browser, which predominately features Google search front and center.

Pichai was also instrumental in orchestrating the exclusive agreements with Apple. Google and Apple first partnered in 2002, but renegotiated in 2016. Pichai was the lead negotiator who made sure Google remained the default search engine on Apple's devices.

Google controls around 90% of the U.S. search engine market. The company is expected to argue that just because it's the default browser on most devices, people aren't forced to use its search. Google says that with just a few clicks and swipes, people can easily switch to another browser — but they choose to stay.

Along with Pichai, Google is planning to call at least 10 other witnesses. The trial is expected to last until the end of November. It's a bench trial, so there's no jury and the presiding judge, Judge Amit Mehta, will give the final ruling.

If Google prevails, the company will likely be able to continue its same business dealings for the time being. If Judge Mehta rules in favor of the Justice Department, it's still unclear how he'd sanction Google. It could be anything from fines to ending the exclusive agreements to company restructuring.

"This has the potential to be precedent setting," Kwoka, the economics professor, said. "So, the stakes are high for everybody."