The colonists had a sovereign ally in the USA Revolutionary War. It was France.

France helped train the colonists to form their own military to carry out the revolution against the British. There were still crowns in Europe at that time, so there was a clear advantage for France to move against the British crown. France was settling land in the "New World." So, France became an ally. The colonists didn't achieve independence on their own.

The Continental Army was established to carry out the war for independence. Sure they were citizens, sure they were a citizen's militia, but, they were not an unprepared citizen to be awaken from their sleep to fight. Their battles started before the Declaration of Independence was signed in Philadelphia. There was reason for those signatories to believe they had a good chance of winning their freedom. This wasn't some kind of childish blood oath based on faux hopes. They knew their potential.

There has been an organized central military concept from the beginning.

The Continental Forces were celebrated on a US Postal Stamp. 

These are some of their officers (click here).

The Continental Army, (click here) Continental Navy, and Continental Marines were created in close succession by the Second Continental Congress in order to defend the new nation against the British Empire in the American Revolutionary War.

These forces demobilized in 1784 after the Treaty of Paris ended the War for Independence. The Congress of the Confederation created the United States Army on 3 June 1784, although the Army's founding is celebrated as occurring on 14 June 1775. The 1787 adoption of the Constitution gave the Congress the power to "raise and support armies," "provide and maintain a navy," and to "make rules for the government and regulation of the land and naval forces," as well as the power to declare war and gave the President of the United States the responsibility of being the military's commander-in-chief.