This is in the shady region between probability and ASB, as it involves geological and cosmological events that obviously didn't happen, but could, theoretically, such as in Stirling's Peshawarverse.
On Christmas Day, 1914, as British and German soldiers play football between their trenches, they look up and stop to see an incredible fireball shoot through the sky. Seconds/minutes later, a single asteroid with a 0.4 kilometer radius impacts directly in London with an explosive yield of 1.4 gigatons.
As some extremely serious butterflies do their thing, the Yellowstone supervolcano erupts 30 minutes later. Although this is a "small" eruption for a supervolcano, it throws *gazillions of tons of debris into the atmosphere and renders a 600 mile radius around the volcano uninhabitable.
What happens next? Note: if the asteroid impact and/or eruption are going to end civilization due to their magnitude, reduce them respectively until civilization in the Northern Hemisphere survives, if tenuously: the point is to cripple the United Kingdom and the United States.