A militant atheist is one who is hostile towards religion. They differ from moderate atheists because they have the desire to propagate atheism and also hold religion to be harmful. Militant atheism was an integral part of the French Revolution, Soviet Union, Cultural Revolution, and is expresses itself today in the ideas of the New Atheist authors.
Militant atheist: All religion does is cause violence! Blah! Blah! Blah!
Believer or Agnostic: You're a militant atheist! What about all of the soup kitchens, hospitals, charities, and orphanages run by beleivers? Are you forgetting about that? Also, did you not take into consideration all the violence and persecution that occurred under the militant atheistic policies under the Soviet Union or Cultural Revolution?