World War I: This is the first day of the Battle of the Somme. Also known as the Somme Offensive, it will last until November 18, 1916, and is one of the largest battles of the entire First World War.
It's also one of the bloodiest military operations in the entire history of humanity.
On this first day, Britain suffers the worst day in its military history with nearly 60,000 casualties — 19,000 killed and 40,000 wounded. Because most of the units are all-volunteer and made up from people from the same locales, this has an awful social impact. Some units are entirely or almost entirely wiped out, which means that in some towns, villages, and areas all men of a certain age range no longer exist.
By the time the Battle of the Somme is over, there will be more than one million casualties total on both sides. There will be extensive criticism later of the generals, especially the British generals, for tactics that lead to such heavy losses.