The answer for this question is: <span>the election of Abraham Lincoln as President
Abaraham Lincoln was always known as someone that really see slavery as something that undermines human's value.
Not long after elected as president, Abraham Lincoln made several policies that make it harder for the southern states to keep their slaves, which led to the first secession.</span>
more people moved into cities, which made them more safe, which meant more people wanted to raise their kids there.
Plague is a deadly disease that people are afraid of.
Cold War
The Cold War was an open yet restricted rivalry that came after World War II between the United States (Western democracies) and the Soviet Union (communist nations) and their respective allies. The Cold War was based on political, economic, and propaganda fronts and had only limited recourse to weapons.
