What crystallized american opinion at the beginning of world war i?
a. great britain's invasion of france
b. russia invasion of germany
c. austria-hungary's invasion of serbia
d. germany's invasion of belgium

D. Germany's invasion of Belgium.

Initially, Americans were reluctant to enter the war. Although a general pro-English, anti-German feeling was the norm, the United States wanted to maintain a policy of non-intervention. However, two events shifted public opinion. The first one were the reports of atrocities in Belgium in 1914. The second one was the sinking of the passenger liner RMS Lusitania in 1915.

c. the passage of the fifteenth amendment