World War I was so deadly primarily because it saw the use of nineteenth-century military tactics with twentieth-century technology. At the beginning of the conflict, the cavalry was still the premier branch of military service, and the commanders believed that this war would be like the last big European fight, the Franco-Prussian War (Prussia was a German principality until that war, when Prussia was able to unite all of Germany into the German Empire, the first time in history that there had ever been a "Germany.") Fought in the same way as the Napoleonic Wars of the early nineteenth century, the Franco-Prussian War taught commanders that offense could still beat defense; in other words, an attacking army could still out-maneuver an enemy on the defensive.
The American Civil War had shown that this was not always true; the combination of more accurate guns, more powerful artillery and the mobility of railroads made defenders far stronger than attackers in many battles. In fact, the Battle of Petersburg near the end of that war showed what entrenched soldiers fighting a defensive battle could do; it took nine months for Union troops under Ulysses S. Grant to break through the Confederate lines, and then only at a huge cost. European observers scoffed at these lessons, however, and believed that a similar situation would never occur in more civilized Europe. They were to be proved very, very wrong, at the cost of millions of lives.
World War I introduced machine guns, modern artillery and airplanes to the battlefield. Railroads made the supply of vast, stationary armies possible, and even the taxi cabs of Paris were employed to bring men to the front in 1914. Horses were removed from the battlefield except as beasts of burden, and tanks entered service in 1916. But the most destructive weapon of World War I was invented in DeKalb, Illinois in 1874 to help cattle farmers keep control of their flocks. Farmer Joseph Glidden invented a useable form of barbed wire after seeing an example at a county fair. Initially used to fence large sections of the American West, during World War I barbed wire was strung by the mile in front of the opposing trenches. As soldiers from one army charged across the shell-cratered hell of No Man's Land, they would become tangled in the wire, easy pickings for the machine gunners in the opposing trench. The massive artillery barrages that characterized the first years of the war were aimed primarily at cutting the enemy's wire, a job at which they failed utterly.
Before World War I, there were no machine guns. Instead, hand-cranked Gatling guns could fire hundreds of rounds quickly, and riflemen had to reload after every shot. By 1914, however, gas-driven, water-cooled machine guns had been perfected. Now a two-man team could fire hundreds of rounds per minute. Carefully placed machine guns could command the entire front of a trench line, and when combined with barbed wire to slow or stop attacking troops, machine guns ruled the battlefield.
Airplanes were also used for the first time in battle. World War I started only eleven years after the Wright brothers had flown the first power-driven airplane at Kitty Hawk, yet aviation had made great strides during that time. Airplanes were initially used only for reconnaissance, replacing hot-air balloons. Quickly each side sent their own planes to deny the enemy any advantage through the air, and thus the Flying Circus was invented. The most famous pilot of the war was Manfred von Richthofen, better known as the "Red Baron." Famously parodied in the Charlie Brown cartoons, Richthofen's record of 80 planes shot down was no laughing matter to the Allies. When Richthofen was killed in 1918, the Allies gave him a full military funeral and dropped leaflets with pictures of the ceremony over German lines to inform the Germans of their actions. The era of chivalry died hard, even after four years of bloody warfare.
The most feared weapon of the war was poison gas. First used by the Germans at the Second Battle of Ypres on 22 April 1915, chlorine, mustard and other gases were used to try to break the stalemate of trench warfare. The effects of gas were gruesome; chlorine caused the lungs to break down and choke a victim to death; mustard gas blinded its victims, and other forms of gas caused the skin to burn and nerves to seize. The most famous of the many poems to come from the trenches, Wilfred Owen's Dulce et Decorum Est, speaks of the horror of watching a friend fail to get his gas mask on in time. Gas never managed to create a decisive advantage for either side, and its use diminished after 1916.
Another technology aimed at breaking the brutal stalemate on the Western Front involved yet another twentieth-century invention: the automobile. Desperate to break through German wire, the Allies built the first primitive tanks in 1915 and 1916, but their use did not become widespread until 1917. There were some successes and many failures and the tank was not the decisive weapon its backers thought it would be. However, during the Allied summer offensive of 1918, which eventually won the war, the tank played an increasingly large role and was instrumental in breaching the German lines.
Armies and air forces were not the only branches of service to undergo technological revolutions prior to and during World War I. Navies were what got Europe into the war in the first place. That is, the naval arms race between Britain and an upstart Germany that embarked on a major naval building spree at the end of the nineteenth century was one of the main causes of the war. In 1906, Britain launched HMS Dreadnaught, the largest battleship ever to that date. It was massive and it revolutionized naval ship building for all time. Submarines were also perfected before and during the war, and they played a major role by bringing the United States into the war when Germany declared renewed unrestricted submarine warfare in February 1917.
Other technologies that had a major impact on World War I were wireless telegraphy, invented by Italian Guglielmo Marconi in 1910, that allowed communication with ships at sea; radio and telephone that allowed communications over land; effective battlefield medicine that cut death rates markedly from previous wars; and powerful artillery guns, one of which—the famous German Big Bertha—was moved on railroads and could fire a shell the size of a VW Bug over 70 miles. None of these weapons proved decisive, and all of them managed to increase the death toll to unheard-of levels. Only with the effective development of the German blitzkrieg of World War II would offense once again become stronger than defense, allowing for a war of maneuver. During World War I, the world learned the high price of too much firepower and not enough mobility.