The Americans almost blew Franco-American relations and a proper end to the Bicentennial. And they probably would have if President Reagan hadn't gone around at the Ottawa economic summit in July casually saying "Y'all come" to the leaders of Canada, Germany, England, France and others who had a historic interest in the way things turned out here 200 years ago.

Suddenly it became clear that French President Franc,ois Mitterrand was not only coming, but that he and his countrymen considered the American Revolution up there in the same league historically with Bastille Day.

What followed was some very high-level panic over how everyone could forget that the French had played a major role in the War for Independence. For one thing, the French gave the colonists about $150 million in aid -- the equivalent of their own annual budget. For another, they suffered more battlefield casualties at Yorktown than the American colonists did.

The state of Virginia, of course, was doing its part in putting together the pageant, though there is some thought that the end of the Bicentennial may have slipped the mind of its junior U.S. senator. Five years ago he was just plain John Warner, head of the Bicentennial Commission, which put on a whale of a show for everybody, but in so doing used up all of its federally allocated funds.

About a month ago President Reagan got into the act by asking Washington investment banker William H.G. FitzGerald to form the Yorktown International Bicentennial Committee (not to be confused with the Virginia-based Yorktown Bicentennial Committee). And did he ever, hitting up some of the biggest names in American and French banking, investment and business communities -- names like Willard Butcher, B.F. Saul, Milton Drewer, James Larkin, Paul Le Blanc, Jean Riboud and Pierre Gosseland, representing such organizations as Chase Manhattan, American Express, Rothschild, Schlumberger and the Franco-American Chamber of Commerce. Neither did FitzGerald overlook some old Reagan standbys like Walter Annenberg and Joseph Coors.

So it is that Ronald Reagan, with a supporting cast of thousands, will have Franc,ois Mitterrand as his "costar" in a documentary film about Yorktown. If you like the pageant, chances are you'll love this $500,000 documentary, which the international committee has hired producer Charles Guggenheim to make.

Not all the support for the film is coming from the committee, which is raising the money to pay for it. The Smithsonian will act as "umbrella" organization to pay the bills, provide historical research and manage the film. Since Secretary of the Army John Marsh considers the Revolutionary War part of U.S. Army history, the Army will provide logistical support. And the White House, under Frank Hodsoll, probable head of the National Endowment for the Arts, will be making sure there's a heavy emphasis on the French connection.

Watching it all with more than spectator interest are WETA, Public Broadcasting and the International Communications Agency, which just might buy a piece of the action and take it on the road. If neither Presidents Reagan nor Mitterrand invented Franco-American relations (credit Lafayette and Ben Franklin for that), schoolchildren probably will think they did when the film hits silver screens in French and American classrooms.

Meanwhile, back at the battle site, the Reagans, Secretary of State Alexander M. Haig Jr. and several key White House aides will go aboard the French frigate de Grasse Sunday for an intimate and classically French lunch hosted by Mitterrand and his wife, Daniele.

For what it's worth, shipmates, "Admiral" Mitterrand doesn't like the increasingly popular calorie-sensitive nouvelle cuisine. He's asking the ship's cook -- or chef, in the French navy -- to rustle up some old-fashioned classical French eats.

As for the first ladies, Nancy and Daniele, meeting for the first time, probably will avoid at least three topics: parties, clothes and politics. The Mitterrands aren't partygoers; Daniele Mitterrand buys French ready-to-wear, and as a member of the Solidarity Committee with the People of Salvador she sympathizes with the Salvadoran leftists.