another life form

I breed like the roach, in like numbers.
Like the roach, I have no natural enemy.
You may crush my children, tear them to shreds,
Drown them, bury them, burn them, flee;
Once they have found you, they come with you ever.

I conquer every hero, rule every world.
I people your visions, govern your thoughts,
Your comings and goings, your givings and takings,
Hem you all round, whatever you do.

I start every war, shield tyrant and butcher,
arrest the poor stranger,
drag him naked to his enemies.

At the last I lay you to rest,
And send forth your mourners,
My song on their lips,
Into the world I have made for you all.

Who am I?