By the mid-19th century what had become a major threat to national unity in the United States ?

would not cooperate with each other voluntarily.

"germany's geographic location was an advantage when it was on the offensive in the war because it was situated next to an extremely wide river, the rhine, which was difficult for the allies to cross."

the answer is true. i hope this .

a agree i agree becuase it is very importnat for example it can tlel you about your past for exmaple   the first culture was the semerians which people are sitll tryignt o uncover there secrets the wall is just anohter treasure that has thousnads of secrets in it to be uncovered