For how long has the US military been involved in defeating insurgencies?

The involvement of the US military in defeating insurgencies spans more than two centuries. This history includes various conflicts where the US has faced insurgent groups, starting from the early days of the country. Significant examples include fighting against Native American tribes during westward expansion, the Philippine Insurrection in the early 1900s, and more recent conflicts in Vietnam, Iraq, and Afghanistan, all of which involved combat against insurgent forces.

The other options underestimate the breadth of the US military's engagement with insurgencies. The timeline of over a century or three decades does not encompass all the significant conflicts and challenges that have involved the military's counterinsurgency efforts throughout American history. Such engagements have shaped military doctrine and strategy, further asserting that this involvement is indeed more than two centuries long.