How come many Americans sided with the Allied Powers even though the United States was neutral?

Because of their pact to always help them if they were ever in a fighting situation. While they sided they only gave them materials it wasn't until Pearl Harbor was bombed by Japan that the U.S. jumped into war.

Explanation:

The gradual German aggression.

Explanation:

The main reason was due to German unrestricted submarine warfare and events like the sinking of the Lusitania. Germany had also sent a telegram to Mexico to exchange a military alliance. British propaganda had been advocated in the US as well showing the war crimes that the Germans committed in Belgium. This forced the US to intervene as the war was at a stalemate.  

no statements but ok.

the statement that best describes the idea of manifest destiny that was used in the 19th century would be that "it was fate that the united states would continue to  expand westward," since people thought the us was "destined" to do so.