Chain of Command begins with a look at the efforts of U.S. and Iraqi forces to regain control of eastern Mosul — the front lines of the war on extremism.

The battle for western Mosul begins, a unit deploys for Afghanistan and the war against extremism stretches to the Caribbean island of Trinidad.

American officials search for dangerous migrants in Colombia; back home, husband and wife Marines prepare for deployment to Afghanistan.

While coalition forces in Iraq gain ground in Mosul, in Trinidad and Tobago, local Muslim leaders are the tip of the spear in the fight against ISIS.

U.S. Marines advise and assist the Afghan army against the Taliban, and one officer watches on Skype as his son is born back home.

Marines continue to work with local forces in Afghanistan, while the U.S. also looks to curb extremism across Africa as well as the jungles of Panama.

The war on terror looks different across the world. At Camp Shorab in Afghanistan, a group of Marines prepare to return home, but are surrounded by reminders of the dangers still haunting the country. Meanwhile in Niger, U.S. Army Special Forces work with local forces to combat extremism, but tragedy strikes when four Americans are killed in an attack. As the losses are mourned, the war against violent extremism continues.