In 1850, most northerners would never have dreamed they would be fighting a war against the south. why did northern public opinion change?

The economic and political differences between the slave states and the free states were brewing since the Louisiana Purchase and getting worse all the time, but it was the rise of the Republican Party and the election of the Lincoln that caused Virginia to finally secede.

I hope this helps :)

a. poison gas was sprayed over battlefields by airplanes to weaken enemy positions before major attacks.