The Canadian government is preparing billions of dollars in retaliatory measures on U.S. exports to Canada if President-elect Donald J. Trump makes good on a threat to impose tariffs on Canadian goods, setting up a potential showdown between two countries that are each other’s largest trading partners.

Canada is assembling a list of measures, including tariffs on U.S. exports to Canada and levies or other restrictions on key Canadian exports to the United States, if Mr. Trump imposes a sweeping 25 percent tariff on Canadian goods.

But the government made clear that it will wait to see what Mr. Trump might do before responding.

“Everything is on the table,” Prime Minister Justin Trudeau said during a news conference in Ottawa, flanked by senior members of his government and the leaders of most of Canada’s provinces and territories. Mr. Trudeau spent most of Wednesday discussing with provincial leaders how to best prepare for potential U.S. tariffs.

For now, Canada’s government is working under the assumption that Mr. Trump is serious about tariffs, which he has linked to the flow of undocumented migrants and drugs across the U.S.-Canada border.

Canada has already announced a series of steps meant to beef up the border, including more personnel and technology. The government on Wednesday said it was also deploy Blackhawk helicopters and more drones to bolster the surveillance of the shared border.

But in more recent weeks Mr. Trump’s has made statements suggesting that his motivation for possible tariffs on Canadian goods may not be solely about border security. The president-elect has repeatedly spoken about Canada becoming another U.S. state, even threatening economic force to annex the country.

Mr. Trump has argued that Canada has a trade surplus with the United States, while the United States provides important defense guarantees to Canada, to make a case that the relationship favors Canada and that the imbalance needs to be addressed.

Mr. Trump’s comments have been met with incredulity and outrage among Canadian political leaders.

Any tariffs would violate a trade agreement among the United States, Mexico and Canada, originally known as NAFTA (North American Free Trade Agreement), which was renegotiated during Mr. Trump’s first presidency. It is scheduled to be renewed next year, but the introduction of tariffs by any country would violate the agreement, potentially rendering it moot.

Mr. Trudeau, on Wednesday, said that Canada was prepared meet any aggressive economic moves by the incoming Trump administration with equal aggression.

“I support the principle of a dollar-for-dollar response,” Mr. Trudeau said. Officials said that this could lead Canada to impose tariffs on hundreds of billions of dollars’ worth of goods exported to the United States.

Canada could impose tariffs on key U.S. imports such as orange juice, officials have said, although Mr. Trudeau on Wednesday acknowledged that tariffs imposed by either government would ultimately hurt both countries and their consumers.

The debate over how to handle the potentially crippling tariffs have strained the relationship between Canada’s federal government in Ottawa, the capital, and its powerful provincial leaders.

While most premiers have united behind Mr. Trudeau and tried to project a “Team Canada” approach, some have struck a contrarian note.

Danielle Smith, the leader of the province of Alberta, which is a key source of oil for the United States, has repeatedly said she would not agree to having her province’s key resource be disrupted, even if targeting oil exports to the United States strengthens Canada’s negotiating hand.

Ms. Smith, who went to Mar-a-Lago to meet Mr. Trump on her own this week, attended Wednesday’s meeting remotely and did not sign a joint statement issued by provincial leaders and Mr. Trudeau. Instead she issued her own statement.

“Federal government officials continue to publicly and privately float the idea of cutting off energy supply to the U.S. and imposing export tariffs on Alberta energy and other products to the United States,” Ms. Smith said in a post on X. “Until these threats cease, Alberta will not be able to fully support the federal government’s plan in dealing with the threatened tariffs.”

Mr. Trudeau, members of his government and premiers from other provinces have made clear that tariffs on oil exports need to remain on the negotiating table as a possible lever to pressure the Trump administration.

About 80 percent of Canada’s oil is exported to the United States, which relies on Canada for more than half of its oil.

If Canada moves to retaliate with its own tariffs, “it has to be fair across the country,” Mr. Trudeau said.

Mr. Trudeau this month announced plans to resign by early March, when his party, the Liberals, selects a new leader who will also become prime minister. The country is expected to head to federal elections soon after, sometime in the spring, putting an essentially lame-duck leader in charge during the first phase of the new Trump administration.

Armed with two garden hoses hooked up to a sputtering tap, Matthew Craig battled fire and smoke to save his house from the onslaught of flames that devastated much of Altadena, a once leafy corner of Los Angeles County. The wind felt like dragon’s breath, he said, and “we were all eating smoke.”

But even though his home is secure, for now, it will be a long time before he and his family feel safe enough to go back. Every room in the house, he said, was covered in ash, dust, soot and dirt that the high winds had blown inside. “My 5-year-old son, wife, dog and I don’t want to be living in a toxic wasteland.”

As residents start returning to neighborhoods ravaged by fire, they are contending with a slew of toxic hazards in the lingering smoke and ash that experts say can cause breathing problems and worse.

When neighborhoods go up in smoke, they burn all manner of human-made materials — vehicles, lead pipes, paint, plastics — which can release toxic smoke and fumes that can linger. A recent study found that even for homes that are spared destruction, smoke and ash swept inside could adhere to rugs, sofas and drywall, creating health hazards that can remain for months.

“There are so many people who have lost their homes in this, and that is devastating,” said Colleen Reid, who studies the health effects of air pollution at the University of Colorado Boulder, and who led the research. “But even the people who feel lucky that their home is fine — they could actually be exposed to lot of toxic materials,” she said. “The wind will get through every crack.”

That was a particular problem, Professor Reid said, because some people may be more likely to move back more quickly or to tackle cleanup themselves if their insurance companies don’t acknowledge this kind of ash and soot damage. And sifting through ash could send hazardous particles back into the air. There was a move in some states to try to force more insurance companies to compensate for smoke and ash intrusion, she said.

Smoke and overall air quality continued to be a risk, said Yifang Zhu, a professor in environmental health at the University of California, Los Angeles. Soot and other fine particles, known as PM2.5, can penetrate the lungs and the heart and can even enter the bloodstream, affecting the entire body.

Ironically, a weakening of the Santa Ana winds that might aid firefighting efforts also meant more smoke would stick around, she said. And air quality readings, which focus on particulate matter pollution like soot, were also not a good indication of the other complex air pollution hazards that the fires had unleashed, she said.

The typical air quality index doesn’t capture volatile organic compounds, for example, that can cause headaches and nausea and are linked to cancer and other illnesses in the longer term. And N-95 masks aren’t as effective against those other pollutants.

“There were so many things that burned, and so many other air toxics,” she said. “That’s why you might smell something, even if air monitors don’t pick it up.”

For people returning to homes in affected neighborhoods, there are steps they can take to protect themselves, she said. Carbon cartridge respirators work better against toxic air pollution than masks. Children and the elderly, as well as people who are pregnant or who have asthma or underlying respiratory or heart conditions, shouldn’t participate in cleanup.

Seek medical attention for any serious symptoms that arise — chest pain, wheezing, heavy coughing — and avoid strenuous exercise. Where possible, stay indoors, and run air purifiers fitted with activated carbon filters, on top of HEPA ones. “It’s just a good idea to be careful,” Dr. Zhu said.

The provisional cease-fire deal announced on Wednesday between Israel and Hamas would begin with an initial phase lasting six weeks, involve the release of 33 hostages and hundreds of Palestinian prisoners and allow the entry of 600 trucks carrying humanitarian relief daily, according to a copy of the agreement obtained by The New York Times.

The agreement requires Hamas to release three female hostages on Day 1, four more on Day 7 and 26 more over the next five weeks, the document says. In exchange, Israel must free multiple Palestinian prisoners for each hostage, including some serving life sentences in some cases, the agreement says.

The 33 hostages to be released in the first phase include women and children, men over age 50 and sick or wounded people. It’s still unclear how many of the 33 are alive, but Israeli officials have estimated that most are.

On the seventh day, Israel would have to withdraw its forces in Gaza to an area in the center of the enclave, and Palestinians displaced in southern Gaza could begin to return to the northern part of the territory, according to the document.

Hundreds of thousands of Palestinians in southern Gaza have been living in tents, makeshift shelters, rented homes and relatives’ apartments for more than a year. Many of those planning to return to the north will most likely find that their homes and neighborhoods have been destroyed, especially residents of Jabaliya, a town in northern Gaza.

The provisional deal says a minimum of 60,000 temporary homes and 200,000 tents would be brought into Gaza during the initial phase.

While the deal calls for 600 trucks to enter daily, United Nations officials have said that increasing the flow of humanitarian aid to that level will be challenging. Israeli restrictions, looting, a shortage of truck drivers and other factors have made it difficult to bring sufficient relief into Gaza, according to the officials. The current number of trucks entering Gaza daily is well below 600.

By the 16th day of the first phase, negotiations about the second phase of the deal — also lasting six weeks — would begin, especially details related to the further exchange of hostages and Palestinian prisoners.

During the second phase, Israel and Hamas would declare a “permanent cessation of hostilities,” Israeli forces would withdraw from Gaza and the remaining living hostages would be traded for Palestinian prisoners, the deal says.

Hamas has long said that it would agree only to a deal that ends the war, but Prime Minister Benjamin Netanyahu of Israel has rejected ending the conflict, indicating he may seek to resume fighting the group after freeing some hostages.

Marco Rubio, the Republican senator from Florida named by Donald J. Trump to be the next secretary of state, was warmly welcomed by senators from both parties at his confirmation hearing on Wednesday. He has served for years on the Foreign Relations and Intelligence Committees in the Senate, and is known as a lawmaker devoted to the details of foreign policy.

“I believe you have the skills and are well qualified to serve as secretary of state,” Senator Jeanne Shaheen, Democrat of Hampshire, said in her opening remarks.

The notable lack of tension at the hearing indicated that Mr. Rubio would almost certainly be confirmed quickly.

Republican and Democratic senators agree on the biggest U.S. challenges.

From the lines of questioning, it was clear what senators want Mr. Rubio and the Trump administration to focus on: China, Russia, North Korea and Iran. Mr. Rubio himself pointed to those four powers — what some call an “axis” — in his opening remarks.

They “sow chaos and instability and align with and fund radical terror groups, then hide behind their veto power at the United Nations and the threat of nuclear war,” he said. As permanent members of the U.N. Security Council, China and Russia have veto power over U.N. resolutions.

Mr. Rubio repeatedly singled out the Chinese Communist Party for criticism, and, unlike Mr. Trump, he had no praise for any of the autocrats running those nations.

He did say the administration’s official policy on Ukraine would be to try to end the war that President Vladimir V. Putin of Russia started, and that leaders in both Kyiv and Moscow would need to make concessions. U.S. officials say Russia has drawn its allies and partners into the war, relying on North Korea for troops and arms, Iran for weapons and training, and China for a rebuilding of the Russian defense industrial base.

Rubio’s stance on Israel and Gaza is firmly within a foreign policy consensus in Washington.

Mr. Rubio defended Israel’s conduct in the war in Gaza, blaming Hamas for using civilians as human shields and calling the deaths of tens of thousands of Palestinians in Gaza, most of them non-combatants, “one of the terrible things about war.”

He expressed concern about threats to Israel’s security. “You cannot coexist with armed elements at your border who seek your destruction and evisceration, as a state. You just can’t,” he said.

When asked whether he believed Israel’s annexing Palestinian territory would be contrary to peace and security in the Middle East, Mr. Rubio did not give a direct answer, calling it “a very complex issue.”

Mr. Rubio’s hearing was about two hours in when the committee’s chairman announced that Israel and Hamas had sealed an agreement to begin a temporary cease-fire and partial hostage release in Gaza. An initial hostage and cease-fire agreement, reached in November 2023, fell apart after a week.

Rubio supports U.S. alliances, despite Trump’s persistent attacks on them.

Mr. Rubio called the North Atlantic Treaty Organization, which Mr. Trump has repeatedly criticized, “a very important alliance” and insisted that Mr. Trump was a NATO supporter. But he also backed Mr. Trump’s argument that a strong NATO requires Europe to spend more money on its collective defense.

The United States, he said, must choose whether it will serve “a primary defense role or a backstop” to a self-reliant Europe.

Though many Senate colleagues praised him, Rubio has detractors.

Some prominent Trump supporters remain distrustful of Mr. Rubio. They recall his vote to certify the 2020 election results despite Mr. Trump’s false claims of election fraud. And they consider Mr. Rubio’s foreign policy record dangerously interventionist.

Mr. Rubio has long been a hawkish voice on national security issues, often in ways that clash with Mr. Trump’s views, even if the ideas are conventional ones among centrist Republican and Democratic politicians.

In the past, Senator Rand Paul, Republican of Kentucky, has criticized Mr. Rubio for advocating aggressive American intervention overseas. Mr. Paul has been outspoken in pushing for less use of U.S. troops abroad and is skeptical about whether economic sanctions can lead to positive outcomes.

On Wednesday, Mr. Paul pointedly asked Mr. Rubio whether he saw any way to work with China rather then persisting in attacks on Beijing, and he also questioned the wisdom of many American and European policymakers who insisted that Ukraine must be admitted to NATO.