The court will most likely act quickly, as TikTok faces a Jan. 19 deadline under a law enacted in April by bipartisan majorities. The law’s sponsors said the app’s parent company, ByteDance, is controlled by China and could use it to harvest Americans’ private data and to spread covert disinformation.

The court’s decision will determine the fate of a powerful and pervasive cultural phenomenon that uses a sophisticated algorithm to feed a personalized array of short videos to its 170 million users in the United States. For many of them, and particularly younger ones, TikTok has become a leading source of information and entertainment.

As in earlier cases pitting national security against free speech, the core question for the justices is whether the government’s judgments about the threat TikTok is said to pose are sufficient to overcome the nation’s commitment to free speech.

Senator Mitch McConnell, Republican of Kentucky, told the justices that he “is second to none in his appreciation and protection of the First Amendment’s right to free speech.” But he urged them to uphold the law.

“The right to free speech enshrined in the First Amendment does not apply to a corporate agent of the Chinese Communist Party,” Mr. McConnell wrote.

Jameel Jaffer, the executive director of the Knight First Amendment Institute at Columbia University, said that stance reflected a fundamental misunderstanding.

“It is not the government’s role to tell us which ideas are worth listening to,” he said. “It’s not the government’s role to cleanse the marketplace of ideas or information that the government disagrees with.”

The Supreme Court’s last major decision in a clash between national security and free speech was in 2010, in Holder v. Humanitarian Law Project. It concerned a law that made it a crime to provide even benign assistance in the form of speech to groups said to engage in terrorism.

One plaintiff, for instance, said he wanted to help the Kurdistan Workers’ Party find peaceful ways to protect the rights of Kurds in Turkey and to bring their claims to the attention of international bodies.

When the case was argued, Elena Kagan, then the U.S. solicitor general, said courts should defer to the government’s assessments of national security threats.

“The ability of Congress and of the executive branch to regulate the relationships between Americans and foreign governments or foreign organizations has long been acknowledged by this court,” she said. (She joined the court six months later.)

The court ruled for the government by a 6-to-3 vote, accepting its expertise even after ruling that the law was subject to strict scrutiny, the most demanding form of judicial review.

“The government, when seeking to prevent imminent harms in the context of international affairs and national security, is not required to conclusively link all the pieces in the puzzle before we grant weight to its empirical conclusions,” Chief Justice John G. Roberts Jr. wrote for the majority.

In its Supreme Court briefs defending the law banning TikTok, the Biden administration repeatedly cited the 2010 decision.

“Congress and the executive branch determined that ByteDance’s ownership and control of TikTok pose an unacceptable threat to national security because that relationship could permit a foreign adversary government to collect intelligence on and manipulate the content received by TikTok’s American users,” Elizabeth B. Prelogar, the U.S. solicitor general, wrote, “even if those harms had not yet materialized.”

Many federal laws, she added, limit foreign ownership of companies in sensitive fields, including broadcasting, banking, nuclear facilities, undersea cables, air carriers, dams and reservoirs.

While the court led by Chief Justice Roberts was willing to defer to the government, earlier courts were more skeptical. In 1965, during the Cold War, the court struck down a law requiring people who wanted to receive foreign mail that the government said was “communist political propaganda” to say so in writing.

That decision, Lamont v. Postmaster General, had several distinctive features. It was unanimous. It was the first time the court had ever held a federal law unconstitutional under the First Amendment’s free expression clauses.

It was the first Supreme Court opinion to feature the phrase “the marketplace of ideas.” And it was the first Supreme Court decision to recognize a constitutional right to receive information.

That last idea figures in the TikTok case. “When controversies have arisen,” a brief for users of the app said, “the court has protected Americans’ right to hear foreign-influenced ideas, allowing Congress at most to require labeling of the ideas’ origin.”

Indeed, a supporting brief from the Knight First Amendment Institute said, the law banning TikTok is far more aggressive than the one limiting access to communist propaganda. “While the law in Lamont burdened Americans’ access to specific speech from abroad,” the brief said, “the act prohibits it entirely.”

Zephyr Teachout, a law professor at Fordham, said that was the wrong analysis. “Imposing foreign ownership restrictions on communications platforms is several steps removed from free speech concerns,” she wrote in a brief supporting the government, “because the regulations are wholly concerned with the firms’ ownership, not the firms’ conduct, technology or content.”

Six years after the case on mailed propaganda, the Supreme Court again rejected the invocation of national security to justify limiting speech, ruling that the Nixon administration could not stop The New York Times and The Washington Post from publishing the Pentagon Papers, a secret history of the Vietnam War. The court did so in the face of government warnings that publishing would imperil intelligence agents and peace talks.

“The word ‘security’ is a broad, vague generality whose contours should not be invoked to abrogate the fundamental law embodied in the First Amendment,” Justice Hugo Black wrote in a concurring opinion.

The American Civil Liberties Union told the justices that the law banning TikTok “is even more sweeping” than the prior restraint sought by the government in the Pentagon Papers case.

“The government has not merely forbidden particular communications or speakers on TikTok based on their content; it has banned an entire platform,” the brief said. “It is as though, in Pentagon Papers, the lower court had shut down The New York Times entirely.”

Mr. Jaffer of the Knight Institute said the key precedents point in differing directions.

“People say, well, the court routinely defers to the government in national security cases, and there is obviously some truth to that,” he said. “But in the sphere of First Amendment rights, the record is a lot more complicated.”

When raging wildfires tore through Pacific Palisades and other local communities this week, they not only left a path of destruction reminiscent of a World War II bombing campaign, but threatened to deepen a crisis that has already left hundreds of thousands of Californians struggling to find and keep affordable homeowners insurance.

The multiple fires from Los Angeles to the San Gabriel Valley that have burned thousands of structures since Tuesday — leading to losses that by one early estimate are well into the tens of billions of dollars — hit Southern California as insurers have been dropping customers statewide citing the increasing number and severity of wildfire-related losses.

The Palisades fire alone, which consumed more than 5,000 homes and structures, is being called the most destructive fire ever to hit the city, while the fires across the county are likely to be one of the most expensive natural disasters in U.S. history.

“It’s just an unmitigated disaster,” said Amy Bach, executive director of United Policyholders, a consumer advocacy group. “Wildfires in January? This just proves insurers’ point that the risk is so significantly increased due to climate change.”

State Farm, the state’s largest home insurer, announced in March it would not renew 72,000 property insurance policies, while Chubb and its subsidiaries stopped writing new high-value homes with higher wildfire risk — just to name two insurers that pulled back from the California market.

It’s not clear how many homeowners in Pacific Palisades and elsewhere might not have had coverage, but at least some homeowners reported that insurers had not renewed their policies before the disaster struck. Actor James Woods, who lost his home in the Palisades fire, tweeted Tuesday that “one of the major insurances companies canceled all the policies in our neighborhood about four months ago.”

State Farm last year told the Department of Insurance it would not renew 1,626 policies in Pacific Palisades when they expired, starting last July.

A spokesperson for State Farm declined to comment on the decision but said: “Our number one priority right now is the safety of our customers, agents and employees impacted by the fires and assisting our customers in the midst of this tragedy.”

The situation has left many homeowners in neighborhoods at high wildfire risk with little choice but to seek relief from the California FAIR Plan, an insurer of last resort that sells policies with lesser coverage. The policies cover losses up to $3 million to a dwelling and its contents caused by certain hazards, such as fire, but do not include personal liability and other protection that are typically offered by private insurers.

The FAIR Plan has seen its policies grow from a little over 200,000 in September 2020 to more than 450,000 as of last September. That has roughly tripled its loss exposure to $458 billion over the same period. Pacific Palisades has one of the state’s highest concentrations of FAIR Plan policy holders, with the insurer estimating its exposure in the neighborhood at $5.89 billion.

JP Morgan analysts estimate that total L.A. County losses could be close to $50 billion, while the losses insurers will have to pay could top $20 billion. Another estimate puts the losses even higher.

Such losses could cause insurers to exit the market completely, which Tokio Marine America Insurance Co. and Trans Pacific Insurance Co. said in April they would do in not renewing 12,556 homeowners.

The losses also could prompt insurers to further raise premiums, even though some insurers already have been granted big rate hikes, such as a 34% increase Allstate received last year.

Denise Rappmund, senior analyst at Moody’s Ratings, said, “These events will continue to have widespread, negative impacts for the state’s broader insurance market — increased recovery costs will likely drive up premiums and may reduce property insurance availability.”

Should insurers further withdraw from the market, that would put additional pressure on the FAIR Plan, which is is backed by the state’s licensed insurers, such as State Farm, who have to pay claims if they exceed the FAIR Plans reserves, reinsurance and catastrophe bonding. The insurers also can assess their own policyholders surcharges in the billions of dollars to bail out the plan under regulations put in place last year by Insurance Commissioner Ricardo Lara as part of his Sustainable Insurance Strategy to help the crippled market.

It’s unclear whether the plan will be able to absorb the losses like it did after the 2018 Camp fire that destroyed the town of Paradise in North California. That conflagration was the single costliest natural disaster in the world that year with $12.5 billion in covered losses and $16.5 billion in total losses, according to the reinsurance firm, Munich RE.

“This further complicates an already complicated and hardened market,” Lara said of the fires, in an interview with The Times.

Nonetheless, Lara’s reforms seek to ensure the FAIR Plan remains solvent and to make it more attractive for insurers to write policies in fire risky neighborhoods now being absorbed by the program. He said the regulations should encourage insurers to write more homeowners policies, and if not, they can be adjusted. “I feel very confident,” he said.

For the first time, California insurers can use so-called “catastrophe models” in setting their rates. Instead of largely relying on past claims data, the computer programs attempt to better refine an insurer’s risk by taking into account a multitude of variables that affect a property’s likelihood to suffer a loss.

The other major policy change allows insurers to charge California homeowners for the cost of reinsurance they buy from other insurers to limit their losses during huge catastrophes, such as wildfires and floods. This cost shift to policyholders is common elsewhere but a big change for California, where it will raise premiums.

In return for those concessions, insurers will have to write insurance in high-risk wildfire neighborhoods equivalent to 85% of their market share, meaning an insurer with a 10% statewide market share would have to cover 8.5% of the homes in such neighborhoods — a target they have at least two years to reach. Lara’s plan has been blasted by the Los Angeles group, Consumer Watchdog, which says the regulations lack teeth in actually requiring insurers to meet the coverage goals.

“The Sustainable Insurance Strategy is not a magic wand. It’s a set of incentives,” Bach said. “At the end of the day, insurers are always still going to analyze, ‘Are we going to make money here or not?’”

How much this week’s fires will disrupt the already troubled insurance market depends, of course, on how big a disaster they are — but all indications are that insurers will have to absorb billions of dollars of claims given the number of homes destroyed, especially in the wealthy enclave of Pacific Palisades, where the average home is valued at about $3.5 million by Zillow.

Insurance industry experts say a clearer picture on the estimated losses will only come after adjusters have time to review submitted claims.

“I think it’s going to be 45 days before we know what the true damage is,” said Max Gilman, president of California personal lines at the brokerage HUB International.

Whatever the final cost, Gilman noted that the fires came after a couple of relatively light fire seasons — though in November the Mountain fire in Ventura County scorched more than some 20,640 acres and destroyed more than 130 homes amid parched conditions. That made it at the time the third most destructive fire in Southern California in a decade.

“I think what’s currently transpiring is going to be of grave concern for the future,” he said. “I feel like we we took three steps forward to take five steps back.”

Denne Ritter, a vice president with the American Property Casualty Insurance Assn. trade group, said it is too early to assess the impact of the fires on Lara’s reforms, especially given how they are just being put in place. Only one catastrophe model has been submitted for review to regulators, while the reinsurance regulation released last month still awaits final approval by the Office of Administrative Law.

“What the insurance industry wants is a healthy market in California where we can compete for business, as we have historically. And the number one priority right now is helping our customers get the resources they need to rebuild their lives and restore their property,” she said.

However, she noted that Mercury Insurance — which recently announced it started writing insurance again in Paradise — and Farmers Insurance, which said last month it is increasing the number of new home policies it will write, have “certainly made moves indicating a more bullish approach on the market.”

Allstate also has said it will resume writing new policies once Lara’s reforms are in place and it can get rates that fully cover its costs.

But all those pronouncements came before this week’s catastrophic fires.