Answer:
1. War and insecurity 2. Economic crisis (poverty) 3. Overpopulation
Explanation:
1. War and insecurity - as example Syrian crisis and war against ISIS
2. Economic crisis (poverty) - mass immigration from Poland to USA in the late XIX and early XX century.
3. Overpopulation - mass immigration to Europe from African countries in the last few decades.
Information in the link below:
https://www.history.com/this-day-in-history/germany-resumes-unrestricted-submarine-warfare
