This article claims that the 1991 Gulf War oil spill is the largest in history: "The Gulf War oil spill, when Saddam Hussein intentionally spilled oil into the Persian Gulf, resulted in anywhere from 240 million to 460 million gallons of crude oil poured into the Gulf. It remains the world’s largest oil spill today."
I was surprised to see that today because just a few weeks ago, while writing the post Humans can be quite stupid, I read this Wikipedia article, which at the time listed the Gulf War oil spill as the third largest in history. The list of the largest oil spills in that Wiki entry has since been updated to include "Kuwaiti oil fires" and "Kuwaiti oil lakes". The Wiki article claims that an incredible 42-63 BILLION gallons of oil were burned in 1991, and a further 1-2 billion gallons of oil were spilled from sabotaged fields in Kuwait that year. One billion gallons is more than five times the amount of oil spilled as a result of the Deepwater Horizon explosion.
Until now I did not know that so much oil was burned in 1991 (more than 200 times the amount spilled by Deepwater Horizon), and I did not know that Saddam's saboteurs discharged millions of barrels of oil directly into the Persian Gulf. The burning of dozens of billions of gallons of oil may not be considered a spill by some, but it's definitely a waste and a huge spike in emissions of carbon dioxide and other pollutants into the atmosphere. Those environmentally damaging acts of sabotage alone should have been reason enough to overthrow Saddam in 1991.