A lot of Japanese people don't think the Americans would help Japan if China were to attack Japan.
Many Japanese people are of the opinion that the American forces would not come to the aid of the Japanese if Japan were invaded by China.
ーA lot of Japanese people don't think the Americans would help Japan if China were to attack Japan.
「多くの日本人は、中国が日本を攻撃したとしても、アメリカは日本を助けないだろうと思っている」
to attack で「攻撃する」
ーMany Japanese people are of the opinion that the American forces would not come to the aid of the Japanese if Japan were invaded by China.
「多くの日本人はもし日本が中国に侵略されたとしても、アメリカ軍は日本を助けに来ないだろうという意見である」
to come to the aid of ...で「…を助けにくる」
to be invaded by ... で「…に侵略される」
ご参考まで！
カナダ 
