The sanctions, which came into effect Monday at midnight east coast time, are the result of the U.S.’s withdrawal from the Iran nuclear deal, which came with a deadline for businesses to stop any business with Iran when it was announced in May. But a counter-measure from the EU came into effect at the same time. It’s a blocking measure designed to protect European countries and to keep at least some part of the nuclear deal alive.

The measure will allow European businesses affected by sanctions to sue the American government in European member state courts, and require European businesses to receive a waiver if they plan to stop activities in Iran. American businesses, by contrast, are required to apply for a waiver in order to continue activities in Iran.

The EU’s defiance of renewed American sanctions represents a serious escalation in tensions between the Trump administration and its European counterparts. EU officials have said the sanctions are “illegal” and vowed to thwart them. In an official statement, Trump promised “severe consequences” for those who violated sanctions, later reiterating the threat on Twitter.

The Iran sanctions have officially been cast. These are the most biting sanctions ever imposed, and in November they ratchet up to yet another level. Anyone doing business with Iran will NOT be doing business with the United States. I am asking for WORLD PEACE, nothing less!

— Donald J. Trump (@realDonaldTrump) August 7, 2018

Russia and China are also among the countries that support the Iran nuclear deal and oppose sanctions. This isn’t the first time the EU has opposed sanctions imposed by the U.S. The same mechanism was used to protect EU countries from sanctions against Cuba.