The reference is to the Spanish Civil War (1936-1939) between the Republican forces, which defended the country's democratically-elected socialist government, and the right-wing Fascist Falange that would eventually win leading to Francisco Franco's thirty-six-year dictatorship.
Derek Walcott's "A Far Cry from Africa" is about a completely different historical event: the Mau Mau uprising against British colonial rule in Kenya (195-1956). The Mau Mau movement was formed by a faction of the Kikuyu tribe, the largest of the country. The revolt was violently repressed by British troops, but, when Kenya gained independence in 1963, its leader, Jomo Kenyatta, was elected the country's first prime minister. The attitude of the poet to the Mau Mau uprising is controversial: writing from the colonial dominion of St. Lucia, he sympathizes against the anti-colonial impulse of the rebellion; yet, he cannot accept the Mau Mau violence, at times directed against their own people who did not share their same views.
The comparison of the Mau Mau uprising to the Spanish Civil War, as yet another example of "a waste of our compassion" is a condamnation of the international community's ineffective attitude. In the case of the Spanish Civil War, European democratic governments decided not to intervene directly to aid the Republican faction not to cause more tensions in Europe. Yet, the Fascist regimes in Italy and Germany supported the Falange which eventually won. The reference is therefore an indictment of those who are said to be horrified by the violence but do nothing or refuse to get directly involved. The Spanish Civil War also recalls in the minds of the readers yet another conflict where both sides committed crimes and brutalities against each other.