He is called a “seargent of imperialism” several times. This is the context behind it (copy pasting from Wikipedia).

First is from the Zulu uprising of 1906 against the British in South Africa:

Gandhi, who was in South Africa at the time, felt that the Indians in South Africa would do best for themselves to serve the British Empire as a reserve force in the Army against the Zulus during the rebellion.[8] Gandhi actively encouraged the British government to recruit Indians. He argued that Indians should support the war efforts in order to legitimise their claims to full citizenship. The British government, however, refused to allow Indians to enlist as combatants. Nonetheless, they accepted Gandhi’s offer to let a detachment of Indians volunteer as a stretcher bearer corps to treat wounded English[9] soldiers. This corps of 21 was commanded by Gandhi. Gandhi urged the Indian population in South Africa to join the war through his columns in Indian Opinion: “If the Government only realised what reserve force is being wasted, they would make use of it and give Indians the opportunity of a thorough training for actual warfare”.[8] Later in 1927 he wrote of the event as “No war but a man hunt”.[10]

Then in the first World War:

In April 1918, during the latter part of World War I, the Viceroy invited Gandhi to a War Conference in Delhi.[82] Gandhi agreed to support the war effort.[83][37] In contrast to the Zulu War of 1906 and the outbreak of World War I in 1914, when he recruited volunteers for the Ambulance Corps, this time Gandhi attempted to recruit combatants.