Militant describes someone who uses aggressive or confrontational methods to fight for a cause they believe in.

A militant protester might block roads or occupy buildings rather than simply holding signs. Militant workers might strike and refuse any compromise with their employers. Labor militants in the early 1900s fought hard for workers' rights, sometimes clashing with police.

Militant doesn't always mean violent, but it does mean forceful and uncompromising. Notice how it differs from passionate: you can be passionate about animal rights while staying peaceful. But if you become militant, you're ready to confront, disrupt, or force others to pay attention.