One in a series of articles recounting Philadelphia's days in the Revolutionary War.

A symphony of cannon and musket fire roars and rattles over the newly green hills of Valley Forge.

Thirteen cannon speak their piece. Then, one after after another, in orchestrated sequence, the soldiers of the Continental Army, drawn up in two lines on the Grand Parade, fire their weapons.

The firing sweeps in a crackling wave down the front line, right to left, then instantly goes back the other way along the rear line.

The exercise is repeated twice.

The date is May 6, 1778.

The Continental Army, after its long winter of deprivation and disease, at last has good news: France has agreed to help the American rebels in their war of independence.

American negotiators in Paris, including Philadelphia's Benjamin Franklin, came to terms with the government of King Louis XVI on Feb. 6, but 18th-century sea travel is so slow that it takes until May for the momentous tidings to arrive.

The aim of the Treaty of Alliance, in its own words, is "to maintain effectually the liberty, Sovereignty, and independence absolute and unlimited of the said united States, as well in Matters of Gouvernement as of commerce."

Both parties pledge "not to lay down their arms, until the Independence of the united states shall have been formally or tacitly assured by the Treaty or Treaties that shall terminate the War."

France, smarting from its defeat in the Seven Years War, sees the alliance as a way to get back at England. Even so, it takes the American victory at Saratoga in October 1777 to convince the French that it is time to go another round with England.

Congress, meeting in York, Pa., since being driven out of Philadelphia by the British, finally gets a copy on Saturday, May 2, of the three documents that make up the pact - a Treaty of Alliance, a Treaty of Amity and Commerce, and an Act Separate and Secret that allows for bringing Spain into the alliance. After taking Sunday off, Congress ratifies all three agreements on Monday.

With French assistance on the way, the future looks much brighter for the American cause, even if a British army does still occupy Philadelphia.

On May 5, a jubilant Gen. George Washington, the commander in chief, who only a few months before had raised the possibility that his army might have to disband, orders his troops to assemble at 9 the next morning.

Each brigade is to listen to its chaplain discourse on God's beneficence in "raising us up a powerful Friend among the Princes of the Earth to establish our liberty and Independence upon lasting foundations."

The religious services are to be followed by a feu de joie, the joyful firing of weapons, interspersed with cheers for the king of France, for European nations friendly to American independence, and for the United States.

"We celebrated the new alliance with as much splendor as the short notice would allow," Washington's aide John Laurens writes on May 7 to his father, Henry Laurens, the president of Congress.

The celebration goes on long after the troops have marched smartly off the parade ground. George Ewing, 24, a junior officer from a New Jersey regiment, writes in his journal that after the ceremony, Washington invited his officers to a "cold Collation... where he did us the honour to eat and drink with us where many patriotic Toasts were drank... with harmless Mirth and jollity."

"Triumph beamed in every countenance," John Laurens writes.

While the officers are drinking their toasts, each soldier in the army is issued a gill - a quarter pint - of rum. Soldiers being held for punishment, including two men who the day before had been condemned to be hanged for desertion, are ordered released.

As if the French alliance weren't enough, Washington finds one more thing to make him happy: the skill and precision with which his troops - who just the year before had to be warned not to dance when they were supposed to be marching - carry out the feu de joie.

In the general orders issued to the army on May 7, Washington praises his men for "the Exactness and order with which their Movements were performed... a pleasing Evidence of the Progress they are making in military Improvement."

Washington - a planter, not a professional soldier - takes none of the credit for the improvement. Instead, he publicly thanks a recently arrived German officer who has been turning the Continental Army into a disciplined fighting force: Friedrich von Steuben, 47, once an aide to the most revered soldier of the day, King Frederick the Great of Prussia.

Steuben, who had never held a rank higher than captain, arrived in America with a recommendation from Franklin and his fellow negotiator in Paris, Silas Deane, describing him as a lieutenant general.

Captain or lieutenant general, Steuben - an almost theatrical figure as he genially flings curses in broken English at the American troops he is training - quickly makes himself indispensable as Washington's drillmaster.

The day after ratifying the French alliance, Congress resolves "that the Baron Steuben be appointed to the office of inspector general, with the rank and pay of major general."

For three years, Washington's army of amateurs has fought alone against Europe's most powerul professional army. American soldiers have shown they have the heart to fight the British, but not always the expertise or the resources.

Now, with a French force on the way to America and European professionals such as Steuben already serving in the Continental Army, they are getting both.

The odds have changed.