The U.S. government and TikTok will go head-to-head in federal court on Monday as oral arguments begin in a consequential legal case that will determine if - or how -- a popular social media platform used by nearly half of all Americans will continue to operate in the country.

Attorneys for the two sides will appear before a panel of judges at the federal appeals court in Washington. TikTok and its China-based parent company, ByteDance, are challenging a U.S. law that requires them to break ties or face a ban in the U.S. by mid-January. The legal battle is expected to reach the U.S. Supreme Court.

The law, signed by President Joe Biden in April, was a culmination of a years-long saga in Washington over the short-form video-sharing app, which the government sees as a national security threat due to its connections to China. But TikTok argues the law runs afoul of the First Amendment while other opponents claim it mirrors crackdowns sometimes seen in authoritarian countries abroad.

In court documents submitted over the summer, the Justice Department emphasized the government's two primary concerns. First, TikTok collects vast swaths of user data, including sensitive information on viewing habits, that could fall into the hands of the Chinese government through coercion. Second, the U.S. says the proprietary algorithm that fuels what users see on the app is vulnerable to manipulation by Chinese authorities, who can use it to shape content on the platform in a way that's difficult to detect.

TikTok has repeatedly said it does not share U.S. user data with the Chinese government and that concerns the government has raised have never been substantiated. In court documents, attorneys for both TikTok and its parent company have argued that members of Congress sought to punish the platform based on propaganda they perceived to be on TikTok. The companies also claimed divestment is not possible and that the app would have to shut down by Jan. 19 if the courts don't step in to block the law.

"Even if divestiture were feasible, TikTok in the United States would still be reduced to a shell of its former self, stripped of the innovative and expressive technology that tailors content to each user," the companies said in a legal brief filed in June. "It would also become an island, preventing Americans from exchanging views with the global TikTok community."

Opponents of the law stress a ban would also cause disruptions in the world of marketing, retail and in the lives of many different content creators, some of whom also sued the government in May. TikTok is covering the legal costs for that lawsuit, which the court has consolidated with the company's complaint and another filed on behalf of conservative creators who work with a nonprofit called BASED Politics Inc.

Though the government's primary reasoning for the law is public, significant portions of its court filings include classified information that has been redacted and hidden from public view. The companies have asked the court to reject the secret filings or appoint a district judge who can ferret through the material, which the government has opposed because it will cause a delay in the case. If admitted into the court, legal experts say those secret filings could make it nearly impossible to know some of the factors that could play a part in the eventual ruling.

In one of the redacted statements submitted in late July, the Justice Department claimed TikTok took direction from the Chinese government about content on its platform, without disclosing additional details about when or why those incidents occurred. Casey Blackburn, a senior U.S. intelligence official, wrote in a legal statement that ByteDance and TikTok "have taken action in response" to Chinese government demands "to censor content outside of China." Though the intelligence community had "no information" that this has happened on the platform operated by TikTok in the U.S., Blackburn said there is a risk it "may" occur.

In a separate document submitted to court, the DOJ said the U.S. is "not required to wait until its foreign adversary takes specific detrimental actions before responding to such a threat."

The companies, however, argue the government could have taken a more tailored approach to resolve its concerns.

During high-stakes negotiations with the Biden administration more than two years ago, TikTok presented the government with a draft 90-page agreement that allows a third party to monitor the platform's algorithm, content moderation practices and other programming. TikTok says it has spent more than $2 billion to voluntarily implement some of these measures, which include storing U.S. user data on servers controlled by the tech giant Oracle. But it said a deal was not reached because government officials essentially walked away from the negotiating table in August 2022.

Justice officials have argued complying with the draft agreement is impossible, or would require extensive resources, due to the size and the technical complexity of TikTok. The Justice Department also said the only thing that would resolve the government's concerns is severing the ties between TikTok and ByteDance given the porous relationship between the Chinese government and Chinese companies.

But some observers have wondered whether such a move would accelerate the so-called "decoupling" between the U.S. and its strategic rival at a time when other China-founded companies, such as Shein and Temu, are also making a big splash in the West. Last week, the Biden administration proposed rules that would crack down on duty-free products being shipped directly from China.

For its part, ByteDance has publicly said TikTok is not up for sale. But that has not stopped some investors, including former Treasury Secretary Steven Mnuchin and billionaire Frank McCourt, from announcing bids to purchase the platform. However, even if such a sale would occur, it would most likely be devoid of TikTok's coveted algorithm, leaving a big question mark on whether the platform would be capable of serving up the type of personally tailored videos that users have come to expect.

The political alignments on the issue are playing out in unconventional ways.

The law, which passed with bipartisan approval in Congress, had encountered resistance from some progressive and Republican lawmakers who voiced concerns about giving the government the power to ban a platform used by 170 million Americans. Former President Donald Trump, who tried to ban TikTok while in office, is now opposing a ban because that would help its rival, Facebook, a platform Trump continues to criticize over his 2020 election loss.

In court, free speech and social justice groups have submitted amicus briefs in support of TikTok, arguing it restricts the First Amendment rights of users and suppresses the speech of minority communities by disrupting a tool many of them use to advocate for causes online. Some libertarian groups with ties to ByteDance investor Jeff Yass have also filed briefs supporting the company.

Meanwhile, the Biden administration has received the backing of more than 20 Republican attorneys general, former national security officials and China-focused human rights groups who are asking the court to uphold the law.