彼らは彼をリーダーとみなした。

they (usually male); them

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

he; him; his; boyfriend

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

leader; reader; reading book; reader (i.e. someone who reads); microreader; dotted line (in printing); leaders; ellipsis mark

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

to consider as; to regard (as equivalent); to deem (as); to equate