Police raided Starbucks' nation headquarters in South Korea on Wednesday, the company confirmed, after a controversial promotional event led to large protests and boycotts of the coffee chain across the country.

"The Seoul police raided the headquarters of Starbucks Korea this morning as part of the ongoing investigation," said a spokesman for the Shinsegae Group, which operates Starbucks Korea.

What was the controversial promotion?

This past May, Starbucks advertised large to-go cups they called "tank tumblers" and seemed to refer to the Gwangju Uprising as "tank day."

On May 18, 1980, the ruling military junta violently put down a pro-democracy protest in the city of Gwangju. At least 165 civilians were killed when the army rolled tanks out into the streets to quell the demonstation.