If you are asking what the general context for the battle of Gettysburg beginning, then the answer is simply the Civil War. If you are asking about a specific incident that caused the battle of Gettysburg to begin, than it would simply be when the Northern and Southern forces began to collide in Gettysburg, PA on July 1, 1863.
Source(s):
http://www.civilwarhome.com/gettysbu.htm


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