What is the role of 美國 in "中國向美國報復"?
This kind of confusion is usually caused by using English grammar to understand Chinese.
In Chinese grammar, 中國向美國報復:
中国， subject
向美国， prepositional phrase worked as adverbial(介词短语作状语); 美国 is the object of the preposition 向.
報復，predicate(谓语)