The main reason World War I was the fourth deadliest war in history was that weaponry had been technologically improved greatly before and during the war, with automatic weapons playing a prominent role. With improvements in defense lacking compared to the advances in weaponry, there was very little a soldier could do to protect himself. Many types of specialized combat were used, such as naval warfare, air combat, trench warfare, and ground warfare. Also, the simple scale of the war was sufficient to cause massive death tolls. With many countries across different continents participating, millions of troops were sent to combat. Nearly 18,000,000 soldiers were killed in the war, and the Spanish Flu killed millions more (Ross 84). The massive dangers facing soldiers was one reason the U.S originally stayed neutral in the war. However, the Zimmermann Telegram provided a reason to go to war that couldn't be dismissed.