Was the US victory in the Revolutionary War inevitable?

The American victory in the Revolutionary War (1775–1783) was not inevitable. Britain was a great power and it was not willing to give up its thirteen colonies.
The key to American victory was the state of George Washington's army. His army was the most important British foe. Had the British succeeded in capturing or annihilating it, the war would have ended. In 1776, Washington was badly beaten in a battle on Long Island. He just managed to escape into New Jersey with the remnants of his army. Instead of finishing him off, the British went into winter quarters.
The two key events that made British victory more unlikely were defeat at Saratoga and the French entry into the war. In 1777, a British army was captured at Saratoga—a huge blow for the British chances. After that, France joined the war on the side of the Americans.
Nevertheless, the British won most of the ensuing battles in the South. The British captured an entire American army at Charleston (1780) in their greatest victory of the war.
In 1781, however, the British made a final and fatal mistake. They stationed their army in Yorktown, Virginia. A French fleet blocked the harbor, and an army under Washington blocked land routes. The British had to surrender and decided to give up the thirteen colonies.