World War I
The pre-1914 Italian army was short of officers, trained men, adequate transport and modern weapons; by April 1915, some of these deficiencies had been remedied but it was still unprepared for the major offensive required by the Treaty of London. The advantage of superior numbers was offset by the difficult terrain; much of the fighting took place high in the Alps and Dolomites, where trench lines had to be cut through rock and ice and keeping troops supplied was a major challenge. These issues were exacerbated by unimaginative strategies and tactics. Between 1915 and 1917, the Italian commander, Luigi Cadorna, undertook a series of frontal assaults along the Isonzo, which made little progress and cost many lives; by the end of the war, Italian combat deaths totalled around 548,000.
In the spring of 1916, the Austro-Hungarians counterattacked in Asiago in the Strafexpedition, but made little progress and were pushed by the Italians back to Tyrol. Although Italy occupied southern Albania in May 1916, their main focus was the Isonzo front which, after the capture of Gorizia in August 1916, remained static until October 1917. After a combined Austro-German force won a major victory at Caporetto, Cadorna was replaced by Armando Diaz who retreated more than 100 kilometres (62 mi) before holding positions along the Piave River. A second Austrian offensive was repulsed in June 1918. On 24 October, Diaz launched the Battle of Vittorio Veneto and initially met stubborn resistance, but with Austria-Hungary collapsing, Hungarian divisions in Italy demanded they be sent home. When this was granted, many others followed and the Imperial army disintegrated, the Italians taking over 300,000 prisoners. On 3 November, the Armistice of Villa Giusti ended hostilities between Austria-Hungary and Italy which occupied Trieste and areas along the Adriatic Sea awarded to it in 1915.

Eastern Front: 

Initial actions: As previously agreed with French president Raymond Poincaré, Russian plans at the start of the war were to simultaneously advance into Austrian Galicia and East Prussia as soon as possible. Although their attack on Galicia was largely successful, and the invasions achieved their aim of forcing Germany to divert troops from the Western Front, the speed of mobilisation meant they did so without much of their heavy equipment and support functions. These weaknesses contributed to Russian defeats at Tannenberg and the Masurian Lakes in August and September 1914, forcing them to withdraw from East Prussia with heavy losses. By spring 1915, they had also retreated from Galicia, and the May 1915 Gorlice–Tarnów offensive allowed the Central Powers to invade Russian-occupied Poland.
Despite the successful June 1916 Brusilov offensive against the Austrians in eastern Galicia, shortages of supplies, heavy losses and command failures prevented the Russians from fully exploiting their victory. However, it was one of the most significant offensives of the war, diverting German resources from Verdun, relieving Austro-Hungarian pressure on the Italians, and convincing Romania to enter the war on the side of the Allies on 27 August. It also fatally weakened both the Austrian and Russian armies, whose offensive capabilities were badly affected by their losses and increased disillusion with the war that ultimately led to the Russian revolutions.
Meanwhile, unrest grew in Russia as the Tsar remained at the front, with the home front controlled by Empress Alexandra. Her increasingly incompetent rule and food shortages in urban areas led to widespread protests and the murder of her favourite, Grigori Rasputin, at the end of 1916.

Romanian participation: Despite secretly agreeing to support the Triple Alliance in 1883, Romania increasingly found itself at odds with the Central Powers over their support for Bulgaria in the Balkan Wars and the status of ethnic Romanian communities in Hungarian-controlled Transylvania, which comprised an estimated 2.8 million of the 5.0 million population. With the ruling elite split into pro-German and pro-Entente factions, Romania remained neutral for two years while allowing Germany and Austria to transport military supplies and advisors across Romanian territory.
In September 1914, Russia acknowledged Romanian rights to Austro-Hungarian territories including Transylvania and Banat, whose acquisition had widespread popular support, and Russian success against Austria led Romania to join the Entente in the August 1916 Treaty of Bucharest. Under the strategic plan known as Hypothesis Z, the Romanian army planned an offensive into Transylvania, while defending Southern Dobruja and Giurgiu against a possible Bulgarian counterattack. On 27 August 1916, they attacked Transylvania and occupied substantial parts of the province before being driven back by the recently formed German 9th Army, led by former Chief of Staff Erich von Falkenhayn. A combined German-Bulgarian-Turkish offensive captured Dobruja and Giurgiu, although the bulk of the Romanian army managed to escape encirclement and retreated to Bucharest, which surrendered to the Central Powers on 6 December 1916.
In the summer of 1917, a Central Powers offensive began in Romania under the command of August von Mackensen to knock Romania out of the war, resulting in the battles of Oituz, Mărăști and Mărășești where up to 1,000,000 Central Powers troops were present. The battles lasted from 22 July to 3 September and eventually, the Romanian army was victorious advancing 500 km2. August von Mackensen could not plan for another offensive as he had to transfer troops to the Italian Front. Following the Russian revolution, Romania found itself alone on the Eastern Front and signed the Treaty of Bucharest with the Central Powers, which recognised Romanian sovereignty over Bessarabia in return for ceding control of passes in the Carpathian Mountains to Austria-Hungary and leasing its oil wells to Germany. Although approved by Parliament, King Ferdinand I refused to sign it, hoping for an Allied victory in the west. Romania re-entered the war on 10 November 1918 on the side of the Allies and the Treaty of Bucharest was formally annulled by the Armistice of 11 November 1918.

Central Powers peace overtures: On 12 December 1916, after ten brutal months of the Battle of Verdun and a successful offensive against Romania, Germany attempted to negotiate a peace with the Allies. However, this attempt was rejected out of hand as a "duplicitous war ruse".

US president Woodrow Wilson attempted to intervene as a peacemaker, asking for both sides to state their demands. Lloyd George's War Cabinet considered the German offer to be a ploy to create divisions among the Allies. After initial outrage and much deliberation, they took Wilson's note as a separate effort, signalling that the US was on the verge of entering the war against Germany following the "submarine outrages". While the Allies debated a response to Wilson's offer, the Germans chose to rebuff it in favour of "a direct exchange of views". Learning of the German response, the Allied governments were free to make clear demands in their response of 14 January. They sought restoration of damages, the evacuation of occupied territories, reparations for France, Russia and Romania, and a recognition of the principle of nationalities. The Allies sought guarantees that would prevent or limit future wars. The negotiations failed and the Entente powers rejected the German offer on the grounds of honour, and noted Germany had not put forward any specific proposals.

Final years of the war: 

Russian Revolution and withdrawal: By the end of 1916, Russian casualties totalled nearly five million killed, wounded or captured, with major urban areas affected by food shortages and high prices. In March 1917, Tsar Nicholas ordered the military to forcibly suppress strikes in Petrograd but the troops refused to fire on the crowds. Revolutionaries set up the Petrograd Soviet and fearing a left-wing takeover, the State Duma forced Nicholas to abdicate and established the Russian Provisional Government, which confirmed Russia's willingness to continue the war. However, the Petrograd Soviet refused to disband, creating competing power centres and causing confusion and chaos, with frontline soldiers becoming increasingly demoralised.
Following the Tsar's abdication, Vladimir Lenin—with the help of the German government—was ushered from Switzerland into Russia on 16 April 1917. Discontent and the weaknesses of the Provisional Government led to a rise in the popularity of the Bolshevik Party, led by Lenin, which demanded an immediate end to the war. The Revolution of November was followed in December by an armistice and negotiations with Germany. At first, the Bolsheviks refused the German terms, but when German troops began marching across Ukraine unopposed, they acceded to the Treaty of Brest-Litovsk on 3 March 1918. The treaty ceded vast territories, including Finland, Estonia, Latvia, Lithuania, and parts of Poland and Ukraine to the Central Powers.
With the Russian Empire out of the war, Romania found itself alone on the Eastern Front and signed the Treaty of Bucharest with the Central Powers in May 1918. Under the terms of the treaty, Romania ceded territory to Austria-Hungary and Bulgaria and leased its oil reserves to Germany. However, the terms also included the Central Powers' recognition of the union of Bessarabia with Romania.