What was the effect of the Declaration of Independence?

There are a number of ways that this could be answered. 
Let us look at two effects this document had, one immediate and one that took longer to
become evident.

First, the Declaration established that the
colonies were truly trying to become independent.  Up until that time, the rebellion had
not explicitly been about independence.  Even after the fighting started at Lexington
and Concord, there were still attempts, such as the "Olive Branch Petition," to
reconcile with Great Britain.  The Declaration made clear that such attempts were over
and that the war was about creating a new country.

In the
longer term, the Declaration's effect has been to serve as a statement of America's
aspirations.  It has helped to define who and what we are as a nation.  By setting out
the idea that our nation is dedicated to the idea that "all men are created equal," the
Declaration has helped to shape our history.  It has given support, for example, to
rights movements that range from abolition to gay rights.  It has served as a national
ideal that we have continued to strive to achieve.

In this
way, the Declaration had important effects both in the short term and in the long
run.