Re: I wouldn't say so. The Declaration of Independence was "The Unanimous Declaration of the Thirteen United States of America." Unless there is something earlier in which the states acted in unison, that would have been the beginning of the American nation.
There was a Declaration of Colonial Rights submitted to the British Parliament.in 1774 by The good people of the several colonies of New Hampshire, Massachusetts Bay, Rhode Island and Providence Plantations, Connecticut, New York, New Jersey, Pennsylvania, New Castle, Kent and Sussex on Delaware, Maryland, Virginia, North Carolina, and South Carolina,
The similarity of the grievances listed in the Declaration of Colonial Rights and the Declaration of Independence is obvious. However, one outstanding difference is that the Declaration of Colonial Rights addresses the British Parliament and the Declaration of Independence addresses King George.
The Declaration of Colonial Rights can be read at:
http://ahp.gatech.edu/declaration_rights_1774.html
Re: "Of course, this might depend on how you define 'American nation.'"
Simply stated, I define the American nation as the United States of America, a Constitutional Federation of Independent and Sovereign Republics.