Not a particular single event, but I noticed one time when looking through “On this day” in Wikipedia that October 19 seems to be a particularly bad day for empirs.
At the end of the 2nd Punic War, after taking major losses at Utica and Great Plains, the Carthaginian’s recalled Hannibal from Italy. Confident in Hannibal’s forces and leadership, they broke the armistice imposed on them after the preceding two defeats and confronted the Romans. Roman general Publius Cornelius Scipio Massinissa. The Carthaginians had a somewhat larger army of 40,000 men and including 80 war elephants compared to the Roman 35,100 men.
The result was disaster for the Carthaginians with 20,000 men killed and 20,000 captured, wiping out the Carthaginian army and ending the 17 year 2nd Punic War.
Three months after the Battle of Castillon, England finally loses the last of its possessions in southern France, thus bringing to an end the Hundred Years war.
With French ships blockading both resupply and evacuation, Lord Cornwallis is forced to surrender to George Washington, ending the battle of Yorktown, the last major battle of the American Revolutionary War and making American independence a fait accompli although it would be another two years before Great Britain officialy recognized that independence.
Napoleon’s invasion of Russia comes to an end as he is forced to begin his retreat from Moscow. This would mark the furthest extent of the French Empire under Napoleon.
Once again, Napoleon faces a major defeat in the battle of Leipzig. The Sixth Coalition, consisting of troops from the Russian Empire, the Austrian Empire, the Kingdom of Prussia, Sweden, and the Grand Duchy of Mecklenburg-Schwerin at 257,000 men outnumbered Napoleon’s 177,000 men. In particular the Sixth Coalition had 1400 guns to Napoleon’s 700. This ended the French Empire’s presence east of the Rhine.
The Austrian Empire, in accordance with the 1866 Treaty of Vienna which ended the Third Italian war of Independence (and a theater of the Austro-Prussian war), handed Veneto and Mantuo to France, which immediately gave them to the new Kingdom of Italy as payback for previous concessions of Savoy and Nice.
Italy, attempting to build itself into an empire, had just invaded Ethiopia, leading to a war that would last until February of 1941. On this date in 1935, the League of Nations would place economic sanctions on Italy which would prove about as effective as such sanctions usually are, that is, not to speak of. While these sanctions caused alarm in Rome, they served to strengthen Musollini’s position as the Italian people saw him as being strong in standing up to the League of Nations and so helped to cement Fascist power in Italy and more strongly drive Italy into the German camp for the coming war.
Not exactly an empire or even a nascent empire (although Iraq did have pretensions of being a leader of the “Arab world”, so perhaps a nascent empire after all) but on October 19, 2005, Saddam Hussein went on trial for Crimes against Humanity.