Four months ago the bicentennial celebration of America's last and decisive victory in the Revolutionary War seemed under siege and close to surrender. Plans were frayed, the budget was nearly busted and the British weren't coming.

Now, on the eve of a four-day program of fireworks, mock battles and speechmaking expected to draw 100,000 people to this 18th Century town on the York River -- including President Reagan, French President Francois Mitterrand and at least one English lord -- the generals in charge are predicting America will also win the battle this time around.

"By the 20th, we'll be all organized," joked state highway department information chief Al Coates, director of publicity for the patriotic extravaganza and one of eight Virginia state officials drafted by Gov. John N. Dalton in July to rescue the affair.

Dalton waited so long to send reinforcements because he hoped Reagan and the federal government would ante up for the show. But after million-dollar appropriation bills offered by Virginia Sen. John Warner and Rep. Paul Trible were killed in Congress this summer, Dalton dispatched his own troubleshooters, known here as the Yorktown Brigade.

"The overall embarrassment about this whole matter is that both during the Carter and now the Reagan administration there has never been any impetus to follow the lead of Virginia and set up a national commission," said Wilton Dillon, an official with the Smithsonian Institution, one of a dozen organizations involved in planning this weekend's bicentennial for the last three years.

Bicentennial officials complain that it has been a struggle to get either the financial backing or the national enthusiasm due the memory of Yorktown, where Lord Cornwallis and 7,500 men, a full quarter of Britain's army in America, were trapped and forced to surrender by American and French troops under George Washington.

After the four-star hoopla accorded the '76 Bicentennial, they say Yorktown is being treated as something of a historical stepchild.

Despite the lack of funding, some confused planning and a regional feud that two weeks ago had neighboring Gloucester County threatening to barricade a bridge into Yorktown, the Virginia Independence Bicentennial Commission has planned a festival complex enough to tax the strategic abilities of a modern-day general.

"I've got 2,000 more troops and entertainers than Washington had in his whole army," said Sam Laudinslager, vice chairman of the Yorktown program committee, who has organized four days of simultaneous events on 13 stages. In a small trailer at the edge of a town parking lot, Laudinslager looked at sheets of program schedules earlier this week with and admitted he was playing the whole weekend by ear. "It's already a zoo out there."

The celebration begins Friday at noon with a show by army parachutists, some appropriate martial music and an address by Dalton. On Monday, ceremonies will end after speeches by President Reagan and French President Mitterrand and a reenactment of the British surrender. Between will be wreath-laying, jets flying in formation, Irish dancing, naval battles on the York River and a few dozen hours of other historic recreations.

An encampment of 5,000 uniformed members of recreated colonial military units will occupy a huge grassy field outside of town. Around that encampment will be hundreds of tents covering concessions and crafts. The four-day affair was designed to mirror the sesquicentennial celebration in 1931.

Ross Weeks, the director of the Bicentennial Commission, says the four-day program has been planned to closely match Yorktown's 150th anniversary celebration in 1931 when President Hoover brought most of official Washington with him for the occasion.

This year, however, there will be no midway attractions or burlesque show. And the organizers have set up an elaborate transportation system to prevent the kind of traffic jam that occurred 50 years ago when40,000 cars drove into this town of a dozen two-lane streets.

"They were still towing cars four days after it was over," Weeks said. His commission has copied the transportation plan, but only the best parts he says, used at Lake Placid, N.Y., for the 1980 Olympic games.

Only a few cars will be permitted into Yorktown this weekend. All others will be directed to 43 parking lots within a 30 mile radius of Yorktown. Four hundred buses have been leased to bring visitors from those lots to town. Another shuttle bus service will be provided within the three-mile square area of bicentennial activities.

Round-trip tickets will cost $5 for adults and $2.50 for children.