Irruption
Part of speech: noun
Definition: The action of irrupting or breaking into; a violent entry or invasion; an intrusion. An abrupt increase of numbers of a particular animal.
Outbreak
Part of speech: verb
Definition: To burst outTo break forth
Part of speech: noun
Definition: An eruption, sudden appearanceA sudden increase.An outburst or sudden eruption, especially of violence and mischief.An uproar, riotA geological layer that breaks out
Example sentence: After the outbreak of war, in April 1940, we left Geneva with our three children aged 4 years, 2 years and 2 weeks only to become part of the disordered refugee crowds fleeing across France from the German army.