Question

Why did King call African Americans' fighting in Vietnam an "irony"?

Answer 1

He was contrasting their role in Vietnam with their lack of rights in the USA.

Explanation:

King saw it as ironic that a significantly high disproportion of American soldiers in Vietnam were fighting for "democracy" on behalf of a country where racial discrimination and segregation was both endemic and institutionalised. In fact in many southern states African-Americans didn't even have the right to vote at this time.