TO AMERICANS it frequently seems that their old friends the French can be exceedingly exasperating and that the alliance has rougher edges than necessary. That's all true, but when these thoughts occur it's useful to recall an earlier moment in that long friendship. When the British surrendered at Yorktown--on this date, 200 years ago --it was because a fleet of 37 French ships of the line had seized control of the Chesapeake and cut off any hope of relief.

In the late summer of 1781, Gen. Cornwallis and his British troops were in well-fortified positions on that long peninsula, with their backs to the sea. They were bored, but hardly worried. Their chief complaint was that they were stuck in a southern swamp hundreds of miles from the center of the action. In New York, the British commander, Gen. Clinton, was preparing for the combined British and French attack on the city that both sides expected to be the decisive battle of the war. Then the allies suddenly changed their strategy.

In early September, a British admiral took his ships down toward Virginia looking for a French squadron, and realized that Adm. de Grasse's main fleet had suddenly appeared from the West Indies. They fought a sharp but inconclusive battle off the Chesapeake Capes and the British, having suffered a good deal of damage, fell back to New York. Meanwhile, the American army under Gen. Washington and the French under Count de Rochambeau had abandoned New York and were cutting rapidly southward.

As the French marched through Philadelphia, then the capital, Count de Rochambeau ordered his men "to salute Congress as a crowned head, and the president as the first prince of the blood." A historian observes that the courtesy must have greatly astonished the congressmen watching from the State House, for they rarely got much respect from their own troops.

Reaching Yorktown, the allies began pressing Gen. Cornwallis back and on Oct. 19, after days of artillery fire, he surrendered. It was a political triumph for the Americans, ending the Revolution, but in military terms it was mainly a French victory. Without de Grasse's ships just over the horizon, the British would have shortly relieved Gen. Cornwallis by sea from New York, and Yorktown would have been one more incident in a long war that might conceivably have come to a different ending.

French warships have returned to the Chesapeake for the celebration of the anniversary, and they are very welcome. That's what anniversaries are for--to remind you how the world was changed, and who was there.