declare war

verb2

1. •
   To make a formal pronouncement, or sign a legal document, that initiates a state of war between one nation and another, usually before hostilities begin.

   • Germany declared war on France, who reciprocated, on August 3 [1939], and England declared war on Germany on August 4, when Belgium was already under invasion. — 2007
2. •
   (figurative) To announce one's intention to fiercely combat or eradicate something.

   • The governor declared war on poverty.