Other forms: militants; militantly
A militant is someone who is engaged in a war or who acts aggressively for their cause. If you are militant in your beliefs, you do not question them any more than a soldier questions his orders.
In the context of undeclared or guerrilla warfare, enemy fighters are often referred to not as soldiers but as militants, because they are not members of a formal army. As an adjective, militant often has a negative feel — people on one side of a debate will often try to cast those on the other side as militants.