From Wikipedia :
"A warlord is a leader able to exercise military, economic, and political control over a subnational territory within a sovereign state due to his ability to mobilize loyal armed forces. These armed forces, usually considered militias, are loyal to the warlord rather than to the state regime. Warlords have existed throughout much of history, albeit in a variety of different capacities within the political, economic, and social structure of states or ungoverned territories."
I don't see the need of a class to play or role play this. Maybe a feat to allow a character of any class to recrute armed force.
Otherwise some people want the warlord to play specific combat feature:Move ally, make ally attack, non-magical healing.
That was good in 4ed, but in 5ed it is clunky.