This cartoon is only tangentially related to the American Civil Rights Movement which was, in any case, essentially done by 1975. Instead, this cartoon has to do with the process of decolonization in Africa. In this cartoon, we see that Africans have been oppressed by colonialism (this is why the man is in chains) but that they are now fighting back. The man is about to knock down the whole system of colonialism like Samson knocks down the temple in the Bible.
This cartoon shows that the process is partially complete (Portuguese Guinea became Guinea-Bissau in 1974, Angola became independent in November of 1975. Rhodesia (later to become Zimbabwe) and South Africa (where apartheid would persist until the 1990s) have still not started to fall.
So, the cartoon is about Africans destroying the system of colonialism.