The British had a pact with the Belgians to protect them in the event of a major conflict. Once the Germans began blazing a path to France through the small country, Britain's hand was forced and their entrance into the first major global conflict became unavoidable. 

The Serbian assassin, Gavrilo Princip, was merely 19 years old when he set the events of World War I into motion. Commonly referred to as an "accidental assassin," Princip only happened to cross paths with the Archduke's car as he was leaving a pub. Acting quickly, he shot both the Archduke and his wife, Sophie. He then attempted to commit suicide by taking a cyanide tablet; however, the tablet was ineffective and Princip would live to see the trickle-down effect of his actions.

By Armistice Day in 1919, 100 countries had become embroiled in the conflict known as World War I. Not only did the 'Great War' result in millions of casualties for these nations, but it also brought the world into the modern era.

Though Iraq had a horrific human rights record and extreme tensions with its Middle Eastern neighbors, and had declared that it would use chemical weapons on Israel in case of military invasion, none of these was the immediate cause of the war. Instead, Iraq’s invasion of Kuwait in August 1990 and subsequent control over its oil fields merited an international response. 

Japanese troops blew up part of the South Manchurian Railway in September 1931, and accused China of orchestrating the incident. Japan used this incident as a pretext to invade Manchuria, which it had gained complete control over by 1932. 

After the invasion, the Chinese turned to the international community for help, and the League of Nations attempted to bring Japan to the negotiation table. By 1932, Japan had ignored international entreaties, withdrawn from the League and established a puppet state of Manchuko. 

After the formation of the Israeli state by the United Nations from lands claimed by the Palestinians, there were decades of conflict. Israelis cracked down hard on Palestinian demonstrations, and also confiscated the land owned by Palestinians. A combination of Israeli land confiscations and the growing Palestinian population created bad conditions for Palestinians, including rising unemployment levels and poor living conditions.

After the Oslo Accords in 1995, Israel committed to withdrawal from some contested areas and to respect the Palestinian right to certain areas of land, but the bloodshed did not stop in subsequent years, and both sides were deeply disappointed with the process. In 2000, Israeli opposition leader Ariel Sharon visited the Temple Mount, which includes the very holy Al Aqsa mosque. This sparked Palestinian riots and quickly turned into widespread demonstrations and violence. 

After independence in 1961, Tanzania has remained relatively peaceful with no instances of large-scale civil conflict. It was involved in the invasion of Uganda and overthrow of Idi Amin but that represents an instance of interstate conflict not civil war. All the other countries listed have experienced at least one civil war since independence.

The Allied Powers are Great Britain, France, Russia, and the United States. Italy could also be included despite having been pledged to Germany (through a treaty) initially. The other options are incorrect because Austria-Hungary (Austro-Hungarian Empire at that time) was part of the Central Powers, which were against the Allies. Japan is incorrect as well because Japan was not involved in World War I.