In the first stanza, Kipling is referring to the Filipinos. In fact the full title of the poem is The White Man's Burden: The United States and the Philippine Islands. Interestingly, Kipling's poem was written in February 1899, the same month and year the U.S. Senate ratified the Treaty of Paris. With the treaty, the United States gained control of the previous Spanish colonies of the Philippines, Guam, Puerto Rico, and Cuba. To gain possession of the Philippines, the United States also had to pay $20 million to Spain.
During the senate debate process, Senator Knute Nelson defended the prevailing notion of American imperialism as a civilizing force. Notably, other public figures disagreed. Some famous anti-imperialists of the time were Mark Twain, William Jennings Bryan, and Grover Cleveland. The notion of American expansionism as a peaceful endeavor is expressed by Kipling in his poem. He argues that the United States must proceed with its global ministry of bringing civilization to foreign nations, regardless of how others perceive their efforts.
To seek another’s profit
And work another’s gain
Take up the White Man’s burden—
And reap his old reward:
The blame of those ye better
The hate of those ye guard—
As history attests, the Philippines thoroughly rejected the American rationale for imperialism. Just two days before the treaty was ratified, Philippine nationalists clashed with American soldiers in a violent conflict. The rebellion evolved into the Philippine-American War, which lasted for three years and left many casualties on both sides. So, in the first stanza, the "sullen peoples/ Half Devil and half child" describe the Filipino people who rebelled against American imperialist rule. Kipling characterizes the Filipinos as savages who are in need of taming.