Coal mines exploded in 1900 and 1924, killing 200 and 172 people respectively. From 1865-1868, the Black Hawk War was fought between the Utes and the Mormons. A similar war, called the Walker War,...
Well for starters WWII was a pretty huge historical event. The Cold War was a pretty significant event too. Those are the two biggest I can think of just off the top of my head.