The only thing that truly ties together all movies in the history genre is inclusion of real-world subject matter that isn't drawn from the current state of affairs.
As the human event that gets recorded in the history books more than any other, war is obviously prime plot material for historical films. Going all the way back to the ancient Greeks and Romans, world history is filled with wars that have been mined as subject matter for adventure movies throughout film history. American history has plenty of its own wars, and all of them, from the Revolutionary war to the war in Afghanistan, have been depicted in film more than once.
Other important moments in history have been fictionalized in film, too, of course. Developments in cultural or scientific history - the history of American music, for example, or the American space program - are common subject matter, and historical biographies are also popular entrants in the genre.
Historical movies can fit within many other secondary genres. Adventure and action movies are common, especially in the case of historical movies based on wars. Romances, dramas and even musicals often fit into the history genre, too, by benefit of the historical periods in which they're set.