What are the most challenging sources of conflict and crisis in the 21st century?

lincoln declared that slavery should not be allowed to spread into the new states, and of course this angered the south at the time.

name two ways organisms join a population and two ways organisms leave a population. two ways organisms join a population are birth rate and immigration. two ways they leave are death rate and emigration.

explanation:

during the hitler era and it caused them to declare war on germany