South Korean police have raided the national headquarters of Starbucks over defamation claims surrounding a controversial marketing campaign. Starbucks Korea had in May launched a coffee tumbler promotion called "Tank Day" on the anniversary of the 1980 Gwangju Uprising, where hundreds were believed to have been killed by military forces. Shinsegae Group, which operates Starbucks in Korea, said it had launched an internal investigation into the campaign and concluded that the mistake was unintentional. In June, Starbucks stores across the country closed for half a day to allow staff to attend history lessons. Shinsegae apologised for the "inappropriate marketing" and said that the Tank Day controversy had led to serious decline in Starbucks sales in South Korea - one of the coffee chain's largest markets.