The song's theme is taken from the Spanish Civil War, and the idealism of Welsh volunteers who joined the left-wing International Brigades, fighting Francisco Franco's military rebels against the Spanish Republic.
The song takes its name from a Republican poster of the time, displaying a photograph of a young child killed by the Nationalists under a sky of bombers with the stark warning "If you tolerate this, your children will be next" written at the bottom.
And yes, before it's said I'm being melodramatic, I know the government isn't bombing anyone. But it's certainly threatening almost all the principles of democracy. And that, surely, in itself is enough to make anyone very, very worried.
Hat tip: Ivan Horrocks