France maintained a strong alliance with Russia because both of these Great Powers feared an expansion of German influence in Central and Eastern Europe. Germany was closely allied with Austria-Hungary, which had long-standing tensions with Russia, due to its domination of Slavic minorities. Similarly, the Ottoman Empire and Russia were often at odds. Great Britain was not governed by an absolutist regime; rather, it was a constitutional monarchy.

Although it is most strongly associated with WWI, trench warfare was used in wars long before, including during the American Civil War. During WWI various technologies were introduced in attempts to break through entrenched defenses, including poison gas (first used in 1915), flame-throwers (also 1915), and tanks (introduced in 1916). Zepplins were used in bombing raids against Allied cities, most particularly London, where several hundred civilians were killed by bombs.

The United States sent a military force of 5,000 men to Northern Russia and another 8,000 to Siberia in the summer of 1918. While the primary objective of these troops was to secure weapons stockpiles, there was also a desire on the part of the Allies to defeat the Bolshevik forces and bring Russia back into the war. Poland did not exist as a country until after WWI. Finland was invaded by the Soviet Union in 1939. Neither China nor Germany had the military strength to mount an invasion of Russia in the summer of 1918.

Europe was, of course, the scene of most of the fighting during the war, but there were also important military engagements in the Middle East, especially between the British and the Ottoman Empire. The Allies launched attacks against German colonies in Africa over the course of the war, and an important battle took place outside the Chinese city of Tsintao, which was controlled by Germany before the war. No military action took place in South America.

Adolf Hitler served in the Bavarian Army during World War I. He was wounded in battle, and gassed during one of the last Allied gas attacks of the war. Roosevelt served as the Secretary of the Navy during the war, while his future allied collaborator, Churchill, served as the First Lord of the Admiralty. Stalin was excused from military service due to the fact that his left arm was crippled by a childhood injury. As the Crown Prince of Japan, Hirohito had military rank, but was (of course) never allowed to fight.

The unification of Germany occurred just as European powers were beginning to create colonies in Africa; thus, Germany was able to gain control of South West Africa (Namibia) and German East Africa (Burundi, Rwanda, and Tanzania). Southeast Asia was dominated by France, Central Asia was largely controlled by Russia (with some British outposts, e.g. in Afghanistan), and the Middle East/West Asia was part of the Ottoman Empire (although Egypt was largely controlled by Britain). China remained an independent state, although the central government was greatly weakened by European and American influences.

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.