Trench warfare during WWI was difficult, with soldiers facing constant wet and cold or hot and dry conditions. They had to watch out for gun shells and gases while also dealing with rats and mice trying to get to their food. Soldiers, engineers, and medics were regularly dying, so new arrivals were common. Over 15 million soldiers were killed during WWI, which began in 1914 due to the assassination of Archduke Franz Ferdinand and saw the first use of chemical weapons like mustard gas.