。
Troops were immediately dispatched to suppress the riot.
Literal
Riot [object-を] suppress [purpose-ために] immediately military [subject-が] was-dispatched.
ために marks purpose ('in order to suppress'). 直ちに is the formal/literary word for 'immediately' — すぐに is the everyday alternative. 派遣された is a passive, putting focus on the troops being sent rather than who sent them — standard for news-style reporting. 鎮圧する (to suppress) is a formal sino-Japanese compound.