Short answers, pulled from the story.
The great man theory is primarily attributed to Thomas Carlyle, a Scottish essayist, historian, and philosopher. He articulated it in a series of lectures on heroism delivered in 1840, later published as On Heroes, Hero-Worship, and the Heroic in History.
Carlyle identified six hero types: divinity (exemplified by Odin), prophet (Muhammad), poet (William Shakespeare), priest (Martin Luther), man of letters (Jean-Jacques Rousseau), and king (Napoleon). Each type represented a different mode of decisive historical influence.
Herbert Spencer argued that attributing historical events to individuals was unscientific. He contended that great men are products of the social environment that formed them, writing that before a man can remake his society, his society must first make him.
In his 1880 lecture "Great Men, Great Thoughts, and the Environment," published in the Atlantic Monthly, William James argued that genetic anomalies in the brains of great men introduce genuinely original influences into their environments. He described these men as social "ferments" whose variations are either preserved or destroyed by their surrounding environment in a process analogous to evolutionary selection.
Frederick Adams Woods provided empirical support in his book The Influence of Monarchs: Steps in a New Science of History. He studied 386 rulers in Western Europe from the 12th century to the French Revolution, examining how those rulers shaped the course of events in their territories.
Ian Kershaw, writing in 1998, argued that Hitler posed fundamental problems for the great man tradition because Hitler's personal attributes were not noble or enriching. Kershaw rejected the theory and instead argued that Hitler's importance derived from how others perceived him, which Kershaw framed through Max Weber's concept of charismatic leadership.