Context : The 61st (2nd South Midland) Division was an infantry division of the British Army raised in 1915 during the Great War as a second-line reserve for the first-line battalions of the 48th (South Midland) Division. 
Subject : 61st (2nd South Midland) Division 
Relation : military branch
What military branch did 61st (2nd South Midland) Division belong to?