Sentence:  Germany declared <WSD>war</WSD> on France, who reciprocated, on August 3 [1939], and England declared <WSD>war</WSD> on Germany on August 4, when Belgium was already under invasion. 
Meaning of the word marked with <WSD> tags:  Organized, large-scale, armed conflict between countries or between national, ethnic, or other sizeable groups, usually involving the engagement of military forces.