President Trump opened a new front in the global trade war on Friday when he threatened a 20% tariff on cars imported from the European Union unless Europe removes import duties and other barriers to U.S. goods, potentially endangering as much as $300 billion in commerce. Trump’s comments sent the European Autos Stocks Index sliding on Friday while shares of Ford Motor and General Motors also dropped.

“Based on the Tariffs and Trade Barriers long placed on the U.S. and it great companies and workers by the European Union, if these Tariffs and Barriers are not soon broken down and removed, we will be placing a 20% Tariff on all of their cars coming into the U.S. Build them here!” Trump tweeted on Friday.

Based on the Tariffs and Trade Barriers long placed on the U.S. & its great companies and workers by the European Union, if these Tariffs and Barriers are not soon broken down and removed, we will be placing a 20% Tariff on all of their cars coming into the U.S. Build them here!

— Donald J. Trump (@realDonaldTrump) June 23, 2018

As a reference, the US currently imposes a 2.5% tariff on imported passenger cars from the EU and a 25% tariff on imported pickup trucks, while the EU imposes a 10% tariff on all imported U.S. cars. Furthermore, in 2017, the US trade deficit with the EU on cars was about $40 billion per year.

As Reuters adds, the U.S. Commerce Department has a deadline of February 2019 to investigate whether imports of automobiles and auto parts pose a risk to U.S. national security, while Commerce Secretary Wilbur Ross said on Thursday the department aimed to wrap up the probe by late July or August with the Commerce Department planning to hold two days of public comments in July on its investigation of auto imports.

Not long after Trump’s tweet, the European Union vowed it was prepared to retaliate immediately should the tariffs be imposed, saying it would continue tit-for-tat escalation in its trade dispute with the U.S. while countering Trump’s assertion that the U.S. is being treated unfairly by the 28-nation bloc.

Speaking to the French Le Monde, Jyrki Katainen, the EU commissioner in charge of jobs and growth, said that “If they decide to raise their import tariffs, we’ll have no choice, again, but to react. We don’t want to fight (over trade) in public via Twitter. We should end the escalation.”

Earlier in the week, the EU imposed retaliatory tariffs on €2.8 billion ($3.3 billion) of American products in response to duties on its metals exports deemed to be a threat to U.S. national security. According to Bloomberg, the commission, the bloc’s executive arm, said that the €1 trillion euros in commerce between the EU and U.S. is equitable, and it disputed Trump’s assertion that the EU needs to be punished because of unfair trade practices.