World War I
Thus, 50 billion gold marks (12.5 billion dollars) "represented the actual Allied assessment of German capacity to pay" and "therefore ... represented the total German reparations" figure that had to be paid.
This figure could be paid in cash or in-kind (coal, timber, chemical dyes, etc.). Some of the territory lost—via the Treaty of Versailles—was credited towards the reparation figure as were other acts such as helping to restore the Library of Louvain. By 1929, the Great Depression caused political chaos throughout the world. In 1932 the payment of reparations was suspended by the international community, by which point Germany had paid only the equivalent of 20.598 billion gold marks. With the rise of Adolf Hitler, all bonds and loans that had been issued and taken out during the 1920s and early 1930s were cancelled. David Andelman notes "Refusing to pay doesn't make an agreement null and void. The bonds, the agreement, still exist." Thus, following the Second World War, at the London Conference in 1953, Germany agreed to resume payment on the money borrowed. On 3 October 2010, Germany made the final payment on these bonds.
The Australian prime minister, Billy Hughes, wrote to the British prime minister, David Lloyd George, "You have assured us that you cannot get better terms. I much regret it, and hope even now that some way may be found of securing agreement for demanding reparation commensurate with the tremendous sacrifices made by the British Empire and her Allies." Australia received £5,571,720 in war reparations, but the direct cost of the war to Australia had been £376,993,052, and, by the mid-1930s, repatriation pensions, war gratuities, interest and sinking fund charges were £831,280,947.

Support and opposition for the war: 

Support: In the Balkans, Yugoslav nationalists such as the leader, Ante Trumbić, strongly supported the war, desiring the freedom of Yugoslavs from Austria-Hungary and other foreign powers and the creation of an independent Yugoslavia. The Yugoslav Committee, led by Trumbić, was formed in Paris on 30 April 1915 but shortly moved its office to London. In April 1918, the Rome Congress of Oppressed Nationalities met, including Czechoslovak, Italian, Polish, Transylvanian, and Yugoslav representatives who urged the Allies to support national self-determination for the peoples residing within Austria-Hungary.
In the Middle East, Arab nationalism soared in Ottoman territories in response to the rise of Turkish nationalism during the war, with Arab nationalist leaders advocating the creation of a pan-Arab state. In 1916, the Arab Revolt began in Ottoman-controlled territories of the Middle East to achieve independence.
In East Africa, Iyasu V of Ethiopia was supporting the Dervish state who were at war with the British in the Somaliland campaign. Von Syburg, the German envoy in Addis Ababa, said, "now the time has come for Ethiopia to regain the coast of the Red Sea driving the Italians home, to restore the Empire to its ancient size." The Ethiopian Empire was on the verge of entering World War I on the side of the Central Powers before Iyasu's overthrow at the Battle of Segale due to Allied pressure on the Ethiopian aristocracy.

Several socialist parties initially supported the war when it began in August 1914. But European socialists split on national lines, with the concept of class conflict held by radical socialists such as Marxists and syndicalists being overborne by their patriotic support for the war. Once the war began, Austrian, British, French, German, and Russian socialists followed the rising nationalist current by supporting their countries' intervention in the war.
Italian nationalism was stirred by the outbreak of the war and was initially strongly supported by a variety of political factions. One of the most prominent and popular Italian nationalist supporters of the war was Gabriele D'Annunzio, who promoted Italian irredentism and helped sway the Italian public to support intervention in the war. The Italian Liberal Party, under the leadership of Paolo Boselli, promoted intervention in the war on the side of the Allies and used the Dante Alighieri Society to promote Italian nationalism. Italian socialists were divided on whether to support the war or oppose it; some were militant supporters of the war, including Benito Mussolini and Leonida Bissolati. However, the Italian Socialist Party decided to oppose the war after anti-militarist protestors were killed, resulting in a general strike called Red Week. The Italian Socialist Party purged itself of pro-war nationalist members, including Mussolini. Mussolini formed the pro-interventionist Il Popolo d'Italia and the Fasci Rivoluzionario d'Azione Internazionalista ("Revolutionary Fasci for International Action") in October 1914 that later developed into the Fasci Italiani di Combattimento in 1919, the origin of fascism. Mussolini's nationalism enabled him to raise funds from Ansaldo (an armaments firm) and other companies to create Il Popolo d'Italia to convince socialists and revolutionaries to support the war.

Patriotic funds: On both sides, there was large-scale fundraising for soldiers' welfare, their dependents and those injured. The Nail Men were a German example. Around the British Empire, there were many patriotic funds, including the Royal Patriotic Fund Corporation, Canadian Patriotic Fund, Queensland Patriotic Fund and, by 1919, there were 983 funds in New Zealand. At the start of the next world war the New Zealand funds were reformed, having been criticised as overlapping, wasteful and abused, but 11 were still functioning in 2002.

Opposition: Many countries jailed those who spoke out against the conflict. These included Eugene Debs in the US and Bertrand Russell in Britain. In the US, the Espionage Act of 1917 and Sedition Act of 1918 made it a federal crime to oppose military recruitment or make any statements deemed "disloyal". Publications at all critical of the government were removed from circulation by postal censors, and many served long prison sentences for statements of fact deemed unpatriotic.
Several nationalists opposed intervention, particularly within states that the nationalists were hostile to. Although the vast majority of Irish people consented to participate in the war in 1914 and 1915, a minority of advanced Irish nationalists had staunchly opposed taking part. The war began amid the Home Rule crisis in Ireland that had resurfaced in 1912, and by July 1914 there was a serious possibility of an outbreak of civil war in Ireland. Irish nationalists and Marxists attempted to pursue Irish independence, culminating in the Easter Rising of 1916, with Germany sending 20,000 rifles to Ireland to stir unrest in Britain. The British government placed Ireland under martial law in response to the Easter Rising, though once the immediate threat of revolution had dissipated, the authorities did try to make concessions to nationalist feeling. However, opposition to involvement in the war increased in Ireland, resulting in the Conscription Crisis of 1918.
Other opposition came from conscientious objectors—some socialist, some religious—who had refused to fight. In Britain, 16,000 people asked for conscientious objector status. Some of them, most notably prominent peace activist Stephen Hobhouse, refused both military and alternative service. Many suffered years of prison, including solitary confinement. Even after the war, in Britain, many job advertisements were marked "No conscientious objectors need to apply".
On 1–4 May 1917, about 100,000 workers and soldiers of Petrograd, and after them, the workers and soldiers of other Russian cities, led by the Bolsheviks, demonstrated under banners reading "Down with the war!" and "all power to the Soviets!". The mass demonstrations resulted in a crisis for the Russian Provisional Government. In Milan, in May 1917, Bolshevik revolutionaries organised and engaged in rioting calling for an end to the war, and managed to close down factories and stop public transportation. The Italian army was forced to enter Milan with tanks and machine guns to face Bolsheviks and anarchists, who fought violently until May 23 when the army gained control of the city. Almost 50 people (including three Italian soldiers) were killed and over 800 people were arrested.

Technology: World War I began as a clash of 20th-century technology and 19th-century tactics, with the inevitably large ensuing casualties. By the end of 1917, however, the major armies had modernised and were making use of telephone, wireless communication, armoured cars, tanks (especially with the advent of the prototype tank, Little Willie), and aircraft.

Artillery also underwent a revolution. In 1914, cannons were positioned in the front line and fired directly at their targets. By 1917, indirect fire with guns (as well as mortars and even machine guns) was commonplace, using new techniques for spotting and ranging, notably, aircraft and the field telephone.
Fixed-wing aircraft were initially used for reconnaissance and ground attack. To shoot down enemy planes, anti-aircraft guns and fighter aircraft were developed. Strategic bombers were created, principally by the Germans and British, though the former used Zeppelins as well.