How did the Declaration of Independence shape America's outlook and conduct during the Revolutionary War?
The Declaration of Independence in my view made the case for separation from England. The declaration focused the revolution on independence rather than seeking redress for Parliamentary bullying. Jefferson's document put an exclamation point on the long fight for self-government, this generally meant taxation. The colonial leaders pressed for representation in Parliament to justify their position to tax the colonies.
Louis Ray Wells' Industrial History of the United States cited the mercantile system and its restriction on trade,had many leading colonial leaders thinking independence as early as 1765.
The Declaration announced that a new nation was ready to step on the world stage. It is a document that has brought people seeking a better life to America for 234 years
I do not believe that the Declaration actually did shape America's conduct during this war. The Declaration was a masterful piece of political writing, but it did not change the way that the colonists fought.
For one example, the colonists, fighting for democracy, accepted help from a power (France) that was much more of an absolute and tyrannical monarchy than England ever had been.
The colonists conduct in the war was shaped by a need to win the war, not by the precepts of the Declaration.
The Declaration of Independence was a document that declared America's sovereignty—as an independent nation—responsible for its own destiny and for guaranteeing individual rights to bear arms and protect their property, as well as to tax themselves.
Once the Colonial government declared its independence, the colonists became members of an independent society and no longer subjects of King George. It made them traitors to the British Crown (as the British government saw it), but it also galvanized them forward to defend their land and rights. The Colonists went head-to-head against British troops, using guerilla warfare to wage a war in their own backyard.
Trent Lorcher, in his article "4th of July Study Guide: the Importance of the Declaration of Independence," states:
[the declaration's] initial importance was limited as a formal declaration of freedom
Whereas many assume the equality and freedoms stated in the Declaration mean freedom from responsibility and work and the necessity for economic and social equality, the creators of the Declaration of Independence intended different: (1) equality under the law; (2) equality in the eyes of God; and (3) freedom from coercion and tyranny, not from work and trials.
The Columbia Electronic Encyclopedia states:
It is essentially a partisan document, a justification of the American Revolution presented to the world; but its unique combination of general principles and an abstract theory of government with a detailed enumeration of specific grievances and injustices has given it enduring power as one of the great political documents of the West.
Of the philosophies that most influenced the document's construction by Thomas Jefferson was John Locke's Second Treatise of Government, as well as George Mason's Declaration of Rights for Virginia. The difference between these documents and those that shaped the philosophical sentiments prevalent in England (by, for example, Thomas Hobbes) was that Locke believed that men had a responsibility to God first and then to each other, whereas the British crown expected unquestioning loyalty to the King, regardless of his (or her) behavior.
Prior to the passage of Declaration of Independence, the colonists were breaking the law by preventing the British from collecting taxes, of searching one's home without due process, or seizing private citizens' possessions. Once the colonists had declared their rights to "live, liberty and the pursuit of happiness," they were no longer, in essence, breaking the law. They had announced to the world that they would defend the rights as put forth in the Continental Congress's declaration, and provided the reasons for the American Revolution when those rights were withheld from the colonists.
I would say that the most profound impact of Jefferson's writing was that it galvanized the Colonial cause for freedom. The way in which the Declaration of Independence was constructed on both the levels of specificity and in an overall sense helped to bring about a newly refocused emphasis on colonial freedom. The document put forth the case for freedom by outlining specific actions on the part of Britain as outlined in the section of Grievances. At the same time, the document also put forth the case for freedom from a human point of view by making the case for colonial freedom not an issue of political rights as much as human rights. In paraphrasing John Locke's concept of inalienable rights, the document did a stellar job in making the case for colonial freedom as a statement of human rights. In universalizing the issue, it brought more colonists more emphatic about freedom from England and the need to go to war. In the end, the document brought to focus the colonial desire for freedom and made the issue one that was undeniable.