Definition of "declare war"

declare war

verb
third-person singular simple present declares war, present participle declaring war, simple past and past participle declared war
1. (of a governmental authority) 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.
  Quotations
  Germany declared war on France, who reciprocated, on August 3 , and England declared war on Germany on August 4, when Belgium was already under invasion.
  2007, Carlos Ramirez-Faria, Concise Encyclopaedia of World History
2. To announce one's intention to fiercely combat or eradicate something.
  Quotations
  Leo Lillard, a Nashville native attending Tennessee State University, had become involved in the sit-ins as soon as they started. It was no longer just the students, it was clear that the town had declared war on racism.
  2011, Henry Hampton, Steve Fayer, Voices of Freedom: An Oral History of the Civil Rights Movement