A crowd gathered along the waterfront in New York City in the summer of 1776. The scene they witnessed was terrifying.

The largest expeditionary force in British history sailed into the American harbor. Over 300 ships brought 32,000 professional soldiers and Hessian mercenaries to crush a rebellion.

Nearby, Gen. George Washington’s army gathered to hear their commander read a document that would forever change the nature of their fight: the Declaration of Independence.

And contrary to how Americans now think of that document – as an inspiring declaration that detailed the grievances of Colonists against the British king and announced their independence from Great Britain – what Washington read to his army was also something else.

The Declaration of Independence was America’s first formal declaration of war. It planted a symbolic flag for Patriots to rally around. It transformed illegitimate rebels without hope of foreign aid into state-sponsored freedom fighters eligible for military alliances.

This foundational American text wasn’t just a philosophical breakup letter but a strategic move to secure vital support for the American war effort. America’s first declaration of war was a high-stakes geopolitical gamble essential to achieving independence.

Converting rebels into soldiers

As I and other military historians show in our forthcoming collection of essays, “America’s First War: The Military History of the Declaration of Independence,” the declaration was written within the confines of 18th-century legal standards that strictly governed diplomacy and warfare.

Thomas Jefferson, the foremost writer of the declaration, relied heavily on the Swiss jurist Emer de Vattel’s 1758 treatise “The Law of Nations.” Vattel stressed the fact that in the eyes of European courts, providing aid to rebels was a violation of sovereignty and a dangerous precedent.

Vattel argued that for foreign powers to intervene legally in conflicts, the oppressed party had to formally declare its independence and assume the status of a state. Jefferson kept Vattel’s treatise open while he was working on the Declaration of Independence to ensure he used the specific terminology required to transform the American........

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