New Delhi, July 12 -- President Donald Trump on Saturday announced he's levying tariffs of 30% against the European Union and Mexico starting Aug. 1, a move that could cause massive upheaval between the United States and two of its biggest trade partners.

Trump detailed the planned tariffs in letters posted to his social media account. They are part of an announcement blitz by Trump of new tariffs aimed at allies and foes alike, a bedrock of his 2024 campaign that he said would set the foundation for reviving a U.S. economy that he claims has been ripped off by other nations for decades.

In his letter to Mexico's leader, President Claudia Sheinbaum, Trump acknowledged that the country has been helpful in stemming the flow of undocumente...