After months of threats, then retractions, followed by more threats, the White House on Thursday pulled the trigger on new steel and aluminum tariffs imposed on Canada, Mexico, and the European Union.

President Trump announced he was “taking action to protect America’s national security from the effects of global oversupply of steel and aluminum.”

In the initial proclamations of a 25 percent tariff on steel imports and a 10 percent tariff on aluminum in March, President Trump welcomed any country with which the United States shares a security relationship to discuss alternative means to address threatened impairment to the national security caused by their steel and aluminum exports to the United States. Thus far, the U.S. has reached an arrangement with South Korea on steel, which was announced on April 30. It has also recently reached arrangements on steel with Australia, Argentina, and Brazil, and with Australia and Argentina on aluminum.

“The United States was unable to reach satisfactory arrangements, however, with Canada, Mexico, or the European Union, after repeatedly delaying tariffs to allow more time for discussions,” read a statement from the Trump administration.

The announcement was widely blamed for a more than 250-point decline on the Dow Jones Industrial Average on Thursday, with traders spooked by the prospect of a multinational trade war. Those fears grew as targets of the steel and aluminum tariffs were issued.

"I am concerned by this decision,” European Commission President Jean-Claude Juncker said. “The EU believes these unilateral U.S. tariffs are unjustified and at odds with World Trade Organization rules. This is protectionism, pure and simple.”

“Over the past months we have continuously engaged with the U.S. at all possible levels to jointly address the problem of overcapacity in the steel sector. Overcapacity remains at the heart of the problem, and the EU is not the source of but on the contrary equally hurt by it. That is why we are determined to work towards structural solutions together with our partners.”

The EC, he said, has “consistently indicated [its] openness to discussing ways to improve bilateral trade relations with the U.S., but have made it clear that the EU will not negotiate under threat.”

“By targeting those who are not responsible for overcapacities, the U.S. is playing into the hands of those who are responsible for the problem,” Junker added. “The U.S. now leaves us with no choice but to proceed with a WTO dispute settlement case and with the imposition of additional duties on a number of imports from the U.S. We will defend the Union’s interests, in full compliance with international trade law."

Commissioner for Trade Cecilia Malmström also weighed in.

“Today is a bad day for world trade,” he said. “We did everything to avoid this outcome. Over the last couple of months, I have spoken at numerous occasions with the U.S. Secretary of Commerce. I have argued for the EU and the U.S. to engage in a positive transatlantic trade agenda and for the EU to be fully, permanently, and unconditionally exempted from these tariffs.”

“Throughout these talks, the U.S. has sought to use the threat of trade restrictions as leverage to obtain concessions from the EU. This is not the way we do business, and certainly not between longstanding partners, friends, and allies,” he added. “We will now trigger a dispute settlement case at the WTO, since these U.S. measures clearly go against agreed international rules. We will also impose rebalancing measures and take any necessary steps to protect the EU market from trade diversion caused by these U.S. restrictions.”

According to the European Commission, the U.S. measures affect exports worth €6.4 billion in 2017, roughly $7.5 billion in U.S. dollars. Europe also plans retaliatory tariffs. In the recent past, it proposed 25 percent tariffs on decidedly U.S. imports, like denim, motorcycles, cranberry juice, and cigarettes.

Canadian Prime Minister Justin Trudeau also railed against the tariff decision. “Let me be clear,” he said. “These tariffs are totally unacceptable.”

“The numbers are clear: The United States has a $2 billion U.S. dollars surplus in steel trade with Canada—and Canada buys more American steel than any other country in the world, half of U.S. steel exports,” he added. “Canada is a secure supplier of aluminum and steel to the U.S. defense industry, putting aluminum in American planes and steel in American tanks. That Canada could be considered a national security threat to the United States is inconceivable.”

Trudeau added that the tariffs “are an affront to the long-standing security partnership between Canada and the United States.”

The Canadian government also announced a slate of potential retaliation efforts. Those measures include tariffs on American goods and products, including its own range of taxes on steel and aluminum.

Mexico is also preparing its response, announcing plans to "impose equivalent measures” on such products as flat steels, lamps, pork, and various fruits and cheeses.