In the united states, slavery played a major role in
the civil war.
the increase in violence among us citizens.
the language and culture of free states.
relations between colonists and american indians.

It's all of the above :

the Civil War.

the increase in violence among US citizens.

the language and culture of free states.

relations between colonists and American Indians.

Explanation:

- the Civil War.

The main cause of the civil war is the southern states' unwillingness to emancipate their slaves. So the president ordered the Union of the northern states to do it by force.

- the increase in violence among US citizens.

After ex-slaves were transformed into US citizens, many people still did not accept the fact that they have to treat the ex-slaves as equal. So violence conducted to the minorities occurred fairly often.

- the language and culture of free states.

In free states, Cultural diffusion occurred between African-American culture and the culture that generally held by Caucasian-American. Example of this would the Harlem Renaissance in New York. when intellectual and artistic development initiated by Black citizens  started to flourish.

- relations between colonists and American Indians

Black people were not the only group enslaved by the colonists. They also enslaved many American Indians and make the relations between the two became extremely hostile.

experiments.

surveys.

questionnaires.

interviews.

case studies.

participant and non-participant observation.

observational trials.

studies using the delphi method.