The French are always involved in momentous occasions in US history. Fighting the French and Indians gave George Washington all the military experience he needed to lead a revolution against Great Britain. A French army and a French fleet at Yorktown secured Cornwallis' surrender and the subsequent independence of the thirteen American colonies from Britain. Thomas Jefferson doubled the land area of the United States in 1803 when he bought the Louisiana Territory from Napoleon (the state of Louisiana's current government is still influenced by the Napoleonic Code, but that's another story). American soldiers and Marines saved Paris from the Germans in 1918, and the American Army and Marine Corps became a modern fighting force in the process. American soldiers landed at Normandy in 1944 and subsequently liberated France from Hitler. The French repaid us by refusing to participate in NATO against the Soviet Union, by leaving a mess in Indo-China that American forces fought for 10,000 days trying to clean up, and by attempting to thwart our current campaign to save the Middle East for democracy.
Today marks another significant day in American-French relations. In the spring of 1862, the American War Between the States was barely a year old and just beginning to get serious. While Union naval forces had scored significant victories, seizing the important port cities of New Orleans and Port Royal, the armies of the Confederate States of America had fought Union land forces to a draw. In March of 1862, Stonewall Jackson executed his brilliant campaign in the Shenandoah Valley, defeating far larger Union forces, and European sympathies began to swing in favor of the Confederacy.
One European power in particular, Napoleon III of France, was openly hostile to the United States. With the US preoccupied with its life and death struggle with the Confederacy, Napoleon III invaded Mexico with the intent of placing a vassal on a Mexican throne and thereby placing himself in position to provide direct aid to the Confederacy. With Mexico's resources, and perhaps Mexican forces, the balance might swing significantly in the Confederacy's favor. The French army landed at Vera Cruz and marched on Mexico City, mimicking Winfield Scott's landing and campaign against the Mexican capital 15 years previous. On May 5, 1862, the French army was annihilated by Mexican forces at the Battle of Puebla. A year later, the Confederacy was weak and the US Army was strong enough to handily defeat Robert E. Lee at Gettysburg. The British were still wavering on the issue of recognizing the Confederacy. The US ambassador to Great Britain warned the British something to the effect of "See that huge modern army and navy we have built to fight our Civil War? If you recognize the Confederacy, we will defeat the rebels and then come get you next."
Interesting, given the current tension with Mexico, that these re-United States owe our existence as an undivided powerhouse to a French defeat at the hands of a Mexican army.