Within a month , much of Europe was in a state of open warfare , resulting in the mobilization of more than 65 million European soldiers , and more than 40 million casualties -- including approximately 20 million deaths by the end of the war .
Within a month , much of Europe was in a state of open warfare . This resulted in the mobilization of more than 65 million European soldiers , and more than 40 million casualties -- including approximately 20 million deaths by the end of the war .