1
general::
Be or cause someone to be opposed to, as in Civil wars often set brother against brother, or The police chief's critics were set against his officers. [Late 1200s] Also see DEAD SET AGAINST.
American Heritage Idioms
2
general::
Phrase(s): set something against someone or something
1. to place or lean something against someone or something. • Dave set the chair against Fred and had to move it away. • I set the rake against the side of the house. 2. to make someone hate or oppose someone or something. • His second wife set him against his former in-laws. • The Civil War set brother against brother.
McGrawhill's American Idioms And Phrasal Verbs