Markets around the globe were shaken. Companies decried the plan. Economists condemned it. Foreign powers threatened to retaliate.

President Donald Trump's plan to impose a 25% tariff on imported steel and a 10% tariff on aluminum has lit a fire. The policy might be made official this week.

Here's how things got to this point:

The buildup

— The first hint that the Trump administration might soon make good on a campaign promise to tax imported steel comes when the Department of Commerce sends several recommendations to the White House. It includes a call for a 7.7% tariff on aluminum and a 24% tariff on steel.

— The department calls on Trump to claim the tariffs are in the interest of national security, saying he could implement them using a little-known provision from a 1962 trade law.

— Corporate America warns Trump that such tariffs could backfire. Business Roundtable, a lobbying group, says the national security justification is a stretch. The group also cautions that it could ignite a trade war if other countries retaliate with tariffs of their own.

— China says it will "take necessary measures to defend our rights" should Trump go forward with the tariffs.

Related: China will use Trump's tariffs to its advantage

Thursday

— Trump makes the bombshell announcement: He wants a 10% tax on all aluminum imports and a 25% tariff on steel.

— US stocks fall sharply, sending the Dow, S&P 500 and Nasdaq down more than 1%. One bright spot, however, is US Steel, which climbs 6%. Domestic manufacturers expect to face less foreign competition if the tariffs are implemented. But steel buyers expect to see their manufacturing costs climb.

Related: What is a trade war? And all your other questions, answered

— The European Union and Canada, which are key US allies, pledge to retaliate if Trump goes through with the plan.

— American steel and aluminum manufacturers praise the policy, but other businesses are not pleased. Many warn that consumers will see prices go up on everything from cars to beer and baseball bats.

Friday

— US investors wake up to find stock markets across the globe rattled by Trump's announcement.

— Reports surface that the tariff announcement sent shockwaves through the White House, where disagreement among aides stirred up a crisis.

— More US trading partners -- including Germany, Australia, and Britain -- voice strong opposition to the tariffs.

— China, the target of frequent attacks from Trump over its trade practices, responds by urging the US "to abide by the multilateral trade rules." How China will react to new tariffs is considered central to whether a trade war will break out.

— International trade experts say they expect the president's decision will make it easier for other countries to enact tit-for-tat tariffs in the name of national security.

— Trump responds to his critics in a tweet declaring that "trade wars are good" and "easy to win." Experts disagree.

— The European Union vows to hit back with tariffs on American-made motorcycles, bourbon, and denim jeans.

— Commerce Secretary Wilbur Ross steps up to defend the president. He says during an interview with CNN that fears of a trade war are "nonsense."

— US stocks regain some of their losses. The Dow ends the day down 71 points.

Related: 'Trade wars are good?' Two words: Great. Depression.

Saturday

— Trump responds to the EU's retaliation threat by promising to slap a new tax on European cars.

— In a tweet, the president blamed "very stupid" trade deals and policies for racking up an $800 billion annual trade deficit. "Our jobs and wealth are being given to other countries that have taken advantage of us for years. They laugh at what fools our leaders have been. No more!" he wrote.

Related: Trump is ready to fight a trade war with Europe, takes aim at car imports

Sunday

— Chinese Vice Foreign Minister Zhang Yesui says the country "does not want a trade war with the United States" but warns it will "not sit idly by and watch China's interests being harmed."

— Amid questions about whether Trump will follow through on his tariff plan, Commerce Secretary Ross tells NBC he has does not believe the president will change his mind. "The president has announced that this will happen this week. I have no reason to think otherwise," he said.

— White House trade adviser Peter Navarro quashes speculation that Trump may exempt some US allies from the tariffs. "There will be no country exclusions," Navarro tells CNN.

--CNN's Julia Horowitz, Nathaniel Meyersohn, Steven Jiang, Yuli Yang and Jethro Mullen contributed to this report.