declare war on

declare war on (someone or something)

1. To officially announce the intention to fight against an enemy country or group in a war. We will have to declare war on the rebels if they advance over our border.
2. To announce the intention to vehemently oppose someone or something. This usage does not refer to physical fighting. The mayor has declared war on crime in our city and is bolstering our police force.
See also: declare, on, war

declare war on

Also, declare war against. Announce one's intent to suppress or eradicate something or someone. For example, The police have declared war on drug dealing in the neighborhood, or Several gangs have declared war against each other. This usage transfers the literal sense of the term, "to state formally one's intention to carry on hostilities against another power," to a somewhat smaller scale.
See also: declare, on, war