Major volcanic eruptions are known to have had significant effects on human history. For example the 1815 eruption of Tambora caused disruption to the weather world-wide which may have contributed to Napoleon’s defeat at Waterloo and certainly caused widespread harvest failures in 1816 the so-called ‘year without a summer’. The 1783 eruption of Laki in Iceland is thought to have directly resulted in about 5 million deaths in Europe and North Africa mostly from famine caused by crop failure and drought, and so may have contributed to the 1789 French revolution. The effect on global weather of the 1600 eruption of Huaynaputina in Peru triggered the Russian famine of 1601-1603 in which about one sixth of the Russian population died and led directly to the downfall of Tsar Boris Godunov.
But are there any collisions with extraterrestrial bodies that have ever had similar widespread affects on the course of history? The 1908 Tunguska impact would certainly have had a very dramatic and long-term effect if it had struck London, Paris, Berlin or St Petersburg rather than a largely inhabited area of Siberia, as would the lesser known 1930 Curuçá River (Amazonas) event, a massive meteoric airbust that again largely went unnoticed as it also occurred over what was essentially uninhabited jungle.
But just to put these two 20th century impact events into context: asteroids with a diameter of 4m enter Earth's atmosphere approximately once per year, while asteroids with a diameter of 7m enter the atmosphere about every 5 years All these objects usually explode in the upper atmosphere and most or all of the solids are vaporized. Asteroids with a diameter of 20m, and which strike Earth approximately twice every century, produce much more powerful airbursts. The 2013 Chelyabinsk meteor was estimated to be about this size and exploded with an airburst of around 500 kilotons which caused injuries to about 1,500 people and $30 million of damage. The Tunguska impact, if indeed it was a meteorite rather than a comet, was probably somewhere around 25-30m in diameter. Something with a diameter of 50m is estimated to strike on average every 750 years and would be expected to explode at an altitude of a 8-10km with an equivalent to about 10,000 Hiroshima bombs. Moreover any asteroid larger than about 30m diameter is liable to strike the ground and produce a significant impact crater and/or tsunami. By contrast to these boulder-sized asteroids, large asteroids with a 1 km diameter strike Earth every 500,000 years on average while collisions with 5 km objects happen approximately once every twenty million years. The last known impact of an object of 10 km or more in diameter was at the Cretaceous–Paleogene extinction event 66 million years ago.
So are there any instances of asteroid or comet impacts that have had significant effects on the course of human history in a manner similar to major volcanic eruptions?