From the Flickr page (from the Duke University Libraries):
Gamble’s photo depicts a sea of people staging a protest with the intent of influencing government action. Following the violent anti-foreign Boxer Rebellion in 1900, imperialist nations imposed great indemnities on China. When the government did nothing to ease the burdens, they lost popular support. After several shifts in political power and little success with overbearing imperialists, student activity increased along with dissension with the government. According to the date of this photo, this particular protest followed the May 4th Movement in 1919 which was an anti-imperialist revolutionary movement. The movement was escalated by the Treaty of Versailles which unexpectedly resulted in Japanese, and not Chinese, control over Shandong, a formerly German controlled region. The apparent betrayal by the Allies fueled nationalism and set a pattern for student movements whose subject matter ranged from unpopular government policies to anti-Japanese demonstrations and even to anti-Christian campaigns. As a result of students’ increased involvement in government affairs, student political activities played a revolutionary role in the social and political changes that occurred during this period. The sign in the left corner of this photo translates to say “Wake up my countrymen” which reflects the mood of Chinese students and their eagerness in ushering in an era of change via these sorts of protests.