The claim that World War I was the deadliest war in history up until then is just one factual discrepancy about the war. Other discrepancies include claims about living in the trenches and the claim that no one actually won the war.
The claim that World War I killed more people than any other in history is false, as the Taiping Rebellion, which broke out in the 1860s, killed between 20 and 30 million. World War I killed 17 million civilians and soldiers.
Another "fact" associated with World War I is that soldiers had to live in the trenches for years, fighting constantly with the enemy. The truth is that the trenches were an awful place to live, with constant exposure to the enemy, dampness and cold. The British Army rotated soldiers in and out of the trenches so that they spent about 10 days a month within the trench system and no more than three days at a time on the front lines.
Another false claim made is that nobody won World War I because the lines simply remained in place for so long. Millions were injured or dead, and the United Kingdom was virtually bankrupt. However, the fact that the Royal Navy trapped the German battleships until the German crews mutinied, and the German army was riddled with holes in its defenses. The German emperor and military leadership said that there was no way out for the German army, leading to the necessity of surrender.