This may sound like irrelevant pedantry, but I think it's really important to remember that the lines written by Martin Niemoller (not really a poem) don't begin "First they came for the Jews." That's because Niemoller's words are a literal description of what happened when the Nazis came to power, and they didn't come for the Jews first. This is what he wrote:
"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."
The reason I point this out is because it's much easier to understand what happened in Nazi Germany if you grasp that the Nazis attacked the communists and socialists and trade unionists first. They were the people who could have put up a fight against Nazism, so Hitler picked them off first. By the time they came for the Jews, opposition to Nazism was severely weakened.
Incidentally, Niemoller is speaking about himself here - he's not writing some kind of sanctimonious parable. The Nazis really did come for him, and, as he says, there was no one left who could have spoken out for him.