January 2007. Revised June 2011.
Thesis statements —the presentation of a thesis in the introduction of a work—can take many forms, so long as they pose a question and offer an interpretive answer.  Though I do not want to confine my students to formulas, here is one that may help them remember the key elements:
Why did [person/persons] [do/say/write something surprising]? [Plausible explanation], but in fact [better or more complete explanation].
Why did Americans reject public housing except as an option of last resort? Given the popularity of homeownership today, one might think that Americans have always insisted on owning their own homes. But in fact, some Americans preferred quality public housing to home ownership, and only strong efforts by the housing industry and conservative politicians foreclosed this option. 
Or, more loosely:
What did Herman Melville mean when he wrote that “warriors/ Are now but operatives”? Official sources, newspaper accounts, and subsequent histories have presented the USS Monitor solely as a symbol of American technological success, but Melville understood machinery’s ability to dehumanize warfare. 
Why did Americans become more concerned about the environment in the decades after World War II? Though a growing concern about the loss of wilderness obviously contributed to the rise of environmentalism, the movement also was a response to environmental change at the edges of the nation”s cities. 
I even found an essay that uses the form almost exactly as it appears here:
How did Syria come to this pass? While some observers see in recent events a parallel with 1989, with the break-up of the East European–style system introduced by the Baathists in the 1960s, this is no velvet revolution, nor is Syria like Jaruzelski’s Poland. The regime’s violence is not ideological. It is far from being the result of an emotional or philosophical commitment to a party that long ago abandoned its agenda of promoting secular Arab republican values and aspirations. The regime’s ruthless attachment to power lies in a complex web of tribal loyalties and networks of patronage underpinned by a uniquely powerful religious bond. 
Let’s take a look at the components of this form:
1. Why? History theses answer the question, why, or, occasionally, how. Who, what, where, and when are important too, but why and how make an argument. 
2. Person/persons. History is about people. Abstract nouns (capitalism, war, society, etc.) are important, but a thesis without people lacks life.
3. Something surprising. The function of any scholarship is to explore the unknown and the mysterious. Challenge yourself with difficult questions.
4. Plausible explanation.
A good way to know that you have formed a good question is if it forces you to choose among interpretations. The question, who wrote “A Utilitarian View of the Monitor”s Fight”? has only one right answer (Herman Melville). The question, why did Melville write “A Utilitarian View of the Monitor”s Fight”? has many plausible answers. Like the second example, the most thorough theses note exactly who believes or believed an alternative explanation.
5. Better or more complete explanation. Not all answers are equally good. Some are plain wrong; they cannot be supported with evidence (Melville was trying to impress Adah Isaacs Menken ). Others account for some, but not all, of the available evidence (Americans were impressed by the power of the Monitor ). The task of a thesis is to show that your explanation explains words or deeds that were not explained before.
Not all good thesis statements need to take this particular form, but most good theses present all of these elements. Show that your argument can explain more evidence than can a rival, and you have yourself a thesis.
 For all their classroom talk of concise thesis statements. academic historians generally spread the statement of their own theses over several paragraphs at the start of an article or several pages of the introduction of a book. Thus, if you want to find a compact thesis statement, you are better off reading historiographical essays or book reviews that summarize the arguments of other works.
Here I have borrowed the idea of a thesis-statement template from the “magic thesis” presented by the UCLA Office of Instructional Development, http://write.oid.ucla.edu/handouts/Thesis_statement_history.rtf (August 8, 2006).
 Adapted from Gail Radford, Modern Housing for America: Policy Struggles in the New Deal Era (Chicago: University of Chicago Press, 1996), 6.
 Adapted from David A. Mindell, “’The Clangor of That Blacksmith”s Fray”: Technology, War, and Experience Aboard the USS Monitor,” Technology and Culture 36 (April 1995), 242-245.
 The second sentence is a direct quotation from Adam Rome, The Bulldozer in the Countryside: Suburban Sprawl and the Rise of American Environmentalism (New York: Cambridge University Press, 2001), 7. It is one of six thesis statements Rome presents in his introduction (pages 7-12), each beginning with the word “though” and taking the form of a weighing of two alternative explanations.
 Malise Ruthven, “Storm Over Syria ,” New York Review of Books. 9 June 2011.
 A remaining question—what is to be done?—is of great interest in other disciplines, but historians answer it only under duress. Others want to change the world; our task is to interpret it.