A collection of important documents in American history with information about each document's author, the historical context of the work, and the significance of the work to US history.
From Credo Reference
"Milestone Visual Documents in American History focuses on visual images as documents to be analyzed and understood in their own right, just as written documents are. Including primary sources from early America to the present day, it features iconic photographs, paintings, cartoons, maps, and more."
From Credo Reference
"A comprehensive source of reference that brings together a wide range of disciplines related to the history and cultures of the United States, from pre-colonial days to the present"
Search a year to see results including major events from that year. More recent years will have "Year in Review" entries - search the year plus a topic or geographic location, then choose the Year in Review option at the bottom of the left sidebar.
A one-stop reference source featuring information from encyclopedias, dictionaries, atlases, and more. Includes images, video, audio pronunciation of terms, maps, data tables, quotations, citation assistance. Subjects include art, biography, quotations, technology, medicine, and more.
Wikipedia has pages for every year starting with 700 BCE. Use these pages to get a general sense of what happened in each year, then do further research on the events and information.
The World History Encyclopedia is an online non-profit resource that has several useful resources for understanding different time periods. These resources include a searchable timeline, maps, and an index organized by time period.
World History: A Comprehensive Reference Set by 
The Hutchinson Chronology of World History by 
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