It derives from a poem:

First they came for the Communists

And I did not speak out

Because I was not a Communist

Then they came for the Socialists

And I did not speak out

Because I was not a Socialist

Then they came for the trade unionists

And I did not speak out

Because I was not a trade unionist

Then they came for the Jews

And I did not speak out

Because I was not a Jew

Then they came for me

And there was no one left

To speak out for me