The technology for guided missiles developed after World War I, but everything else was a part of warfare from World War I on, and some, like the machine gun, had already been used in earlier conflicts. This was the first war that involved the use of aircraft for fighting that also saw the use of poisonous gases (like mustard gas) and tanks. 

The sinking of the Lusitania occurred in 1915, once the war had already started. The Agadir Crisis, a diplomatic incident between Britain and France on one side and Germany on the other over French influence in Morocco, escalated tensions between the Germany and the growing Anglo-French alliance by increasing British and French fears of German aggression. The European naval arms race of the 1890s-1910s was part of the growing rivalry and militarism among the European powers, especially Britain and Germany, that contributed to the growing tensions of the time. The Second Balkan War of 1913 (between several Balkan states and Bulgaria) increased tensions in the Balkans, a troubled region of Europe where continued ethnic tensions in this period factored into the larger European tensions of the period, and would help directly catalyze the outbreak of war in 1914.

The Second World War led to the complete decline of the ability of European nations to maintain overseas colonies. Britain and France were exhausted from financing the war and there was little desire among the populations of either nation to continue their imperial past. India, a country that had supported the British war effort, gained its independence in 1947.

"Lebensraum" means living space. It was Hitler's belief that the German people needed more territory to grow and to prosper, and he felt that as much of Central Europe was settled by German-speaking people, he had a "legitimacy" to expand aggressively into neighboring states. The policy of "Lebensraum" led to the annexation of the Sudetenland and much of Central Europe. 

At the end of World War Two, Europe was divided between a capitalist and democratic West and a communist East. The capitalist nations signed the North Atlantic Treaty Organization to combat the perceived threat of communism and the communist nations countered with the Warsaw Pact.

To date, the United States is the only country known to have used nuclear weapons in a conflict, dropping two bombs on the Japanese cities of Nagasaki and Hiroshima at the end of the Second World War.

The Nuclear Non-Proliferation Treaty was signed to prevent the spread of nuclear weapons to nations that had not already acquired them and to encourage disarmament among those nations that already possessed nuclear weapons. India, Pakistan, and Israel have never signed the treaty, but North Korea originally signed the treaty before announcing its withdrawal in 2003. North Korea is thought to have developed nuclear weapons by 2006, making it the only state to withdraw from the Nuclear Non-Proliferation Treaty and then acquire nuclear weapons.

The Nuclear Non-Proliferation Treaty was signed in 1968. Its primary purpose was to prevent new nations from acquiring nuclear weapons; however, another part of the treaty was to encourage those nations that already had nuclear weapons to disarm. This has been largely unsuccessful, although every nation that signed the treaty has reduced its supply of nuclear weapons.

The full list of countries who possessed nuclear weapons in 2014, in the order they acquired them, reads as follows: The United States (1945), Russia (1949), The United Kingdom (1952), France (1960), China (1964), India (1974), Pakistan (1998), North Korea (suspected 2006), Israel (unknown).

Mutually Assured Destruction (M.A.D.) is the belief that two nuclear powers will not launch a nuclear attack on one another because such an attack would ensure the complete annihilation of both the initial attacker and the defender. Essentially, each country can thought to be saying, "If you nuke me, I will nuke you back." This concept acted as a deterrent during the Cold War between the United States and the Soviet Union.