Few prospects were as feared by German war planners more than the possibility of having to fight a two-front war against both France and Russia. This fear led to the creation of the Schlieffen Plan, by which Germany would invade France via Belgium with the hopes of defeating France quickly before turning to Russia. This invasion would constitute a violation of Belgian neutrality, which was guaranteed by Great Britain, so carrying out the Schlieffen Plan almost guaranteed that Britain would enter the war on the side of France (which it did). Italy did eventually enter the war against the Axis powers, specifically in order to gain territory from Austria-Hungary, but not until 1915. The campaign against the Serbs was fought by Austrian forces, and was not a major concern of Germany.

The Treaty of Brest-Litovsk between Germany and the newly-created Soviet Union was a complete triumph for Germany, which gained extensive territory in the East as well as the withdrawal of Russian/Soviet troops from its Eastern Front. This allowed Germany to turn all of its efforts to defeating the Allied powers in the West. Unfortunately, soon after the treaty was signed, the US began sending fresh troops to the Western Front at a rate of about 10,000 per day. Compounding this was the situation in Germany itself, where the population faced severe food shortages after suffering through over four years of a highly effective British naval blockade. Inspired by the example of the Soviet revolution in Russia, and tired of the war and shortages, various socialist groups and even some military units began to revolt against the war and the German government in the Fall of 1918. Meanwhile, Germany's weaker allies found it increasingly impossible to continue fighting, and so withdrew from combat. All this led to Germany's acceptance of the Armistice that ended the fighting on November 11, 1918.

As a consequence of their work in factories, and other critical economic arenas, women in Great Britain and the US were granted the right to vote immediately after the war. They also gained more freedom to seek employment outside the home in the postwar world. The American military was not desegregated by race until after WWII, despite a history of valorous service among African American (and other) non-white soldiers. Although many Indians hoped that their support for Britain during WWI would result in independence, India did not gain independence until after WWII. Russian serfs had been freed from their legal ties to the land in 1861 (two years before the Emancipation Proclamation). Finally, the voting age in the US was lowered from 21 to 18 by the 26th Amendment in 1971, based on the argument made during the Vietnam conflict that if an 18-year old soldier was old enough to fight for his country, then he was old enough to vote.

While the sinking by Germany of the Lusitania was a serious international incident, by itself it could not overcome a general distaste in the United States in for becoming entangled in a European war. Further, American public opinion was fairly equally divided between pro-German and pro-Allied positions. After the sinking of the Lusitania, the US demanded, and got, a promise from Germany that it would stop sinking passenger ships (restricted submarine warfare). Germany's announcement that it would return to unrestricted submarine warfare (and so go back to sinking any ship it chose) coincided with its efforts to ally itself with Mexico. The Zimmerman Telegram, a part of the negotiation between Germany and Mexico, was intercepted by British Intelligence and published to inflame American public opinion against Germany. It worked, and the US declared war on Germany on April 6, 1917.

In 1914, Archduke Franz Ferdinand of Austria was assassinated by Serbian nationalists in Sarajevo. This prompted Austria to declare war on Serbia. In turn, Germany declared war on Serbia and soon all of Europe was embroiled in one of its goriest conflicts.

By the inception of World War I, the Ottoman Empire was already a shell of itself. Sensing opportunity at the war's outbreak, the Empire entered the war and allied itself with the Germans in 1914. However, the empire was ill-equipped for war, falling to allied forces and withdrawing from the war in 1918. Less than ten years later, the Ottoman Empire ceased to exist at all, much of its territory divided between Russia, England and France.

The Doolittle Raid occurred on April 18, 1942 planned by Lieutenant Colonel James Doolittle. 16 B-25B's dropped bombs on Tokyo and Honshu. Although the raid was of little strategic importance and inflicted minimal damage to Tokyo, it was a huge most for American morale in the months after Pearl Harbor. Moreover, it demonstrated to the Japanese that their home island of Honshu was vulnerable to air attack.

Karl Dönitz, head of the Kriegsmarine (Nazi German Navy) was named as Hitler's successor in his will, making him the leader of the remainder of Nazi Germany from the moment of Hitler's death (April 30, 1945) until the dissolution by the Allies of the remnant of the Nazi regime which had organized themselves into a minor provisional government called the Flensberg Regime. 

X-ray imaging was invented in the late 19th century. While it did see significant advances during the period of the World Wars, its advent predates both of them.

Though all of the developments listed factored into the hellish blend of fire and steel which defined WWI, the machine gun and bolt-action rifle both brought significant power to defending infantry without aiding offensive infantry, allowing a small number of troops to bring large-scale enemy attacks to a screeching halt. Gas inflicted only around 3% of casualties, and though psychological warfare efforts were extensive during the war, they are not credited with majorly effecting morale on either side. Aircraft played a fascinating but non-decisive role in the war, while tanks were introduced too late in the war (1916)  and in insufficient numbers with poor tactics to be a decisive influence on the course of battle.