Sentence:  Germany <WSD>declared war</WSD> on France, who reciprocated, on August 3 [1939], and England <WSD>declared war</WSD> on Germany on August 4, when Belgium was already under invasion. 
Meaning of the word marked with <WSD> tags:  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.