Why are we sending thousands of military personnel to Guam?

Over the next five years, as many as 30,000 service members and their families will descend on the small island of Guam, nearly tripling its presence there. It's part of a larger agreement that the U.S. signed with Japan to realign American forces in the Pacific, but how will this multi-billion dollar move impact the lives and lifestyle of Guam's nearly 180,000 residents? NOW ON PBS travels to the U.S. territory of Guam to find out whether its environment and infrastructure can support such a large and quick infusion of people, and why the buildup is vital to national security.