During the late 19th and early 20th centuries, who decided what it meant to be an American and how did they define it?

Social Dwarnists (one of which is Herbert Spencer)

Explanation:

During the late 19th and early 20th centuries, SOCIAL DWARNISTS decided what it meant to be an American.

SOCIAL DWARNISTS defined what it meant to be Americans as people from the northern and western Europeans only who had come to settle in America earlier.

While other white races from eastern and southern Europe such as Italian and Polish, and Asia like Chinese and Japanese were considered to be lesser Americans

the decreasing neutrality of the u. s. in world war i

hope this : )

increasing defense spending.

explanation: