August 1983 will be remembered as the month France went to war and nobody hung around to find out what happened.

When 180 crack French paratroopers, operating under the ironic euphemism of "instructors," stormed into Chad on Aug. 9 to thwart the assault of Libyan-backed rebels, the streets of Paris were nearly deserted. Phones rang incessantly in empty offices in the defense and foreign ministries. Occasionally, an incredulous secretary would deign to reply with the lilting, melodic refrain: "conge annuel."

There is no more sacrosanct ritual in France than the summer vacation season. Every Aug. 1 weekend, the great, surging exodus from Paris creates a tangle of traffic jams toward the sea and the mountains. One month later, the flow is reversed as the vacation saga concludes with "la rentree" in early September.

Unfortunately, the wicked never rest, and the sight of an entire country or continent going on vacation in August sometimes suits nefarious schemes.

Adolf Hitler put the finishing touches on his blitzkrieg blueprint in August 44 years ago just before the army of the Third Reich smashed across the Polish border on Sept. 1.

The Soviet Union decided that the third week in August was a propitious time to squelch Czechoslovakia's flirtation with a free society when, 15 years ago, Russian tanks rumbled into the streets of Prague to topple Alexander Dubcek's government before the rest of Europe awoke from a vacation snooze.

This August, Chad erupted on the global map as the season's most volatile East-West flashpoint. Suddenly, France, as the former colonial master of the landlocked desert nation, was exhorted by President Reagan to exert control in its "sphere of influence" in order to block the ostensible designs of the Soviet Union, whose Libyan surrogate, Col. Muammar Qaddafi, was employing his own surrogates, the rebel forces of former Chad President Goukouni Oueddei, to seize vast, sandy tracts of north and west Africa.

With everybody away on vacation, French President Francois Mitterrand was left to his own devices. Ever since the days of De Gaulle, France's policy toward its African colonies has been run by presidential fiat out of the Elysee Palace.

Now, with opposition politicians no longer around to snipe at him, and even his prickly allies, the communists, off gobbling up oysters in Brittany, Mitterrand dominated the stage as the rest of the world, especially the Americans, waited with bated breath to see what he would do.

Mitterrand's critical decisions were made well out of the public eye. He held a secret meeting with Reagan's ambassador at large, Vernon Walters, a former deputy director of the Central Intelligence Agency, at his country home in Laatche, in southwest France.

Mitterrand shrewdly kept his own counsel as he slowly increased French forces in Chad throughout August. Once they had formed a defensive bulwark to shield the southern half of Chad from further advances by the rebels, Mitterrand grandly unveiled his "carrot and stick" strategy, designed to encourage new peace negotiations, in a meticulously crafted interview appearing in the pages of Le Monde.

During those weeks of suspense, only the most purposeful leaks dripped out of government circles. In retrospect, Mitterrand's subtle maneuvering over Chad was greatly abetted by the absence of French officialdom. Nobody was around to share pieces of the great game plan with frustrated reporters.

The president's top adviser on African affairs, Guy Penne, was unavailable. He was dispatched on a tour of African capitals to explain France's ambiguous policy on Chad to other former colonies.

Penne's assistant was none other than the president's son, Jean Christophe Mitterrand, who undoubtedly was privy to information about the unfolding crisis. Mitterrand junior happens also to be a former journalist. But as is the wont of many who cross the line from journalism to government service, he studiously avoided the press, except for a few trustworthy friends.

Meanwhile, in the capital's half- deserted streets and in the scattered cafes that dared stay open in August, people seemed afflicted with ennui over another violent chapter in Chad's long civil war.

The result, at least so far, has produced probably the most approving consensus that the beleagured French socialists have earned for any single policy or event they have administered during two years in power.

We should have realized that Mitterrand knows his people and his country well enough to take advantage of the great August void. One of his campaign promises in 1981 was to extend the annual vacation for every Frenchman from four weeks to five if elected to the presidency. He won. And he did.