In February 1778, a Prussian soldier called Baron Friedrich von Steuben arrived at Valley Forge. He convinced Washington that he could train the Continental Army in European military formations and bayonet charges. By late spring, Steuben had created a disciplined fighting force. The Marquis de Lafayette, a young French soldier, also spent part of the winter at Valley Forge. Fired with enthusiasm for the revolution, Lafayette had joined Washington's staff as a major general without pay.

France's entry into the Revolutionary War in 1778 forced Great Britain to defend the rest of its empire. The British expected to fight the French in the West Indies and elsewhere, and so they scattered their military resources. As a result, Britain no longer had a force strong enough to battle the Americans in the North.

In 1778 General Sir Henry Clinton succeeded General Sir William Howe as chief commander in America. With fewer resources than his predecessor, he could accomplish practically nothing in the north. In June 1778, he evacuated Philadelphia, with the intention of concentrating his forces in New York. Washington, overtook him at Monmouth, New Jersey, and in an action on June 28 both armies suffered about equal loss. The Battle of Monmouth was the last major Revolutionary War battle in the North. Thereafter (except for the winter of 1779 at Morristown) Washington made West Point on the Hudson the headquarters of his army, but Clinton felt he was too weak to attack him there.

A noteworthy movement, in 1778-1779, was the expedition of George Rogers Clark, under the authority of the state of Virginia, against the British posts in the northwest. With a company of volunteers Clark captured Kaskaskia, the chief post in the Illinois country, on July 4, 1778, and later secured the submission of Vincennes, which, however, was recaptured by General Henry Hamilton, the British commander at Detroit.

Washington hoped to drive the British from New York City in a joint operation with the French. In July 1778, a French fleet under Admiral Charles Hector d'Estaing reached America. But the French warships were unable to cross a sandbar at the mouth of New York Harbor. Later that summer, a combined French and American effort to take Newport, R.I., also failed. In November, d'Estaing sailed southward to protect the French West Indies from British attack.

Problems along the western frontier also troubled Washington in 1778. That year, Loyalists and Iroquois Indians massacred frontier settlers in Pennsylvania and New York. Washington sent Major General John Sullivan to take revenge in 1779. Patriot troops burned Iroquois villages and destroyed crops. Many Iroquois starved to death as a result.

On December 29 of 1778, Colonel Archibald Campbell with an expeditionary corps of 3,500 men from Clinton's army in New York, captured Savannah, Georgia, defeating the American force under General Robert Howe (no known relationship to British General Sir William Howe). In the following month he pushed into the interior and occupied Augusta.