The most reductive answer to this question is 1871, when Germany as a country came into existence on the basis of a humiliating defeat of France in the Franco-Prussian War. Not only was there no such thing as a country called "Germany" before then, but the harsh feelings and especially territorial losses the nascent Germany inflicted on France (Alsace-Lorraine) made a desire for "revanche" or revenge against Germany a dominant factor in French politics until the Franco-German rivalry boiled over into World War I (in conjunction with English-German naval race tensions, Balkan nationalism, and a host of other factors.
But I think a more interesting answer to this question, allowing for some ambiguity in the term "Germany," is October 14, 1806. That's when Napoleon's Grande Armée encountered the Prussian army in a pair of battles in east-central Germany. The French won both the Battle of Jena, where Napoleon's 40,000-man army defeated 53,000 Prussians, and the Battle of Auerstadt, where Napoleon's "Iron Marshal" Nicolas Davout, in a tour de force, crushed more than 60,000 Prussians with his single corps of 27,000 men.
In the aftermath of Jena and Auerstadt, Napoleon swiftly captured Berlin, then defeated the Prussians' Russian ally to triumph in the War of the Fourth Coalition. The Treaties of Tilsit that concluded that war were very generous to Russia, but devastating to Prussia, which lost half its territory, had to pay a huge war indemnity, was reduced to a veritable French puppet state, and subject to occupation by French garrisons, who Prussians were required to support.
Unsurprisingly (and now we move from the broad strokes of history into my particular expertise), the Prussians didn't take this well. When the fortunes of war turned and they found themselves part of the victorious coalition to defeat Napoleon in 1814 and again in 1815, the Prussians (or at least, some prominent Prussians) were widely noted in Europe for their vehement anti-French attitude. While Britain and Austria wanted to punish France for the war, cut her down to size a little bit, and prevent any future French aggression, Prussians advocated (particularly in 1815) for dismembering French territory; their soldiers were particularly brutal when occupying France after Waterloo; the Prussians even tried to blow up the Parisian bridge named after the battle of Jena as symbolic revenge for that humiliating loss. (They were finally talked out of it by some combination of the Duke of Wellington, Russian Tsar Alexander I, and the restored French King Louis XVIII, who is reported to have threatened to sit on the bridge himself to keep it safe.)
The Prussians largely didn't get any of the big punishments they wanted of France, though the French felt plenty aggrieved by the treatment they received at the hands of the Prussians (especially in those parts of northern France that the Prussian army occupied). And though more than 60 years would pass between Jena-Auerstadt and the Franco-Prussian War, it clearly had not been forgotten. The headline of the New York Times on Sept. 4, 1870, after Napoleon III's defeat by the Prussian-led armies at the Battle of Sedan, was "JENA AVENGED." The 5 billion-franc war indemnity France was required to pay in the Franco-Prussian War's peace treaty was explicitly designed to be proportional, based on population, to Prussia's 1807 indemnity.
The Franco-Prussian War, as we've already discussed, created the German Empire (ruled by the Prussian royal family) and shifted the now Franco-German rivalry into a new gear. Both sides would engage in an arms race, a diplomatic struggle for position, and constant military espionage (with famous consequences in the Dreyfus Affair) for decades to come, with the tug-of-war over Alsace-Lorraine the festering sore at the heart of the issue. But arguably the emotional root of the entire struggle goes back to Napoleon and Davout at Jena and Auerstadt.