The phrase "to join the fight" means to actively participate or get involved in a battle, conflict, or struggle, to support or defend a cause. Full definition
You, too, can join the fight against breast cancer. (health.com)
Because today, people all over the globe are taking their future into their own hands and joining the fight for climate solutions in their own back yards. (climaterealityproject.org)
In this expansion, the fearless French army joins the fight in a brutal defense of their homeland. (leveldowngames.com)