The Noemvriana (Greek: Νοεμβριανά, "November Events") of December [O.S. November] 1916, also called the Greek Vespers, was a political dispute, rooted in Greece's neutrality in World War I, that escalated into an armed confrontation in Athens between the Greek royalist government and the Allies.

Tensions began early in the war, and escalation started in May 1916, where the Greek fortress of Roupel was surrendered to the Central Powers, mainly Bulgarian forces, raising concerns among the Allies of a secret alliance between Greek government and the Central Powers. This potential alliance threatened the Allied forces bivouacking in Thessaloniki since late 1915. Diplomatic negotiations between King Constantine I and the Allies took place throughout the summer, with the king advocating for Greek neutrality, which would favor the Central Powers. The Allies, however, demanded demobilizing the Greek army and surrendering war materials lost at Roupel to ensure Greece’s neutrality. However, the failed negotiations by the end of the summer and the Bulgarian Army's advance in Macedonia led to a military coup by Venizelist officers in Thessaloniki. Supported by the Allies, former Prime Minister Eleftherios Venizelos established a provisional government in northern Greece to form an army to reclaim territories lost to Bulgaria, thus splitting Greece into two factions.

The involvement of the Hellenic army alongside the Allies and the country's division sparked anti-Allied protests in Athens. In late October, King Constantine and the Allies reached a secret agreement, but pressure from his military advisers forced the king to abandon it. On 1 December [O.S. 18 November] 1916, the Allies landed a small contingent in Athens to enforce their demands but met with organized resistance. An armed confrontation ensued until a temporary compromise was reached. The next day, after the Allied forces evacuated, royalist mobs rioted in Athens, targeting Venizelos' supporters. The violence lasted three days and became known as the Noemvriana, named after the November date in the Old Style calendar. This event deepened the social rift between the Venizelists and royalists, which became known as National Schism.

After Noemvriana, the Allies intensified efforts to remove King Constantine I. A naval blockade was imposed, isolating royalist areas, leading the population of Athens to starve. Following Constantine's abdication in June 1917, his son, King Alexander, ascended the throne and, and Greece was united under Venizelos' leadership. Greece then joined the Allies in World War I, providing the numerical superiority in the Macedonian front. By 1918, the Greek army helped secure an Allied victory in the Balkans, contributing to the liberation of Serbia and the defeat of the Central Powers.

Greece emerged victorious after 1912–1913 Balkan Wars, almost doubling her territory. The unstable international political climate of the early 20th century placed Greece in a difficult position. The ownership of the Greek-occupied eastern Aegean islands was contested by the Ottoman Empire, which claimed them as its own. In the north, Bulgaria, defeated in the Second Balkan War, was engineering revanchist strategies against Greece and Serbia. The assassination of Archduke Franz Ferdinand of Austria in Sarajevo precipitated Austria-Hungary's declaration of war against Serbia. This caused Germany and Austria-Hungary, and countries allied with Serbia (the Triple Entente) to declare war on each other, starting World War I.

Greece, like Bulgaria, initially maintained neutrality during the conflict. The Greek leadership was divided between Prime Minister Eleftherios Venizelos, who supported Great Britain on the side of the Allies, and King Constantine, who was educated in Germany and married to the Kaiser's sister. The king admired Prussian militarism and was anticipating a quick German victory. The king wanted Greece to remain neutral in the conflict, a strategy favorable to Germany and the Central Powers.

In early 1915, Britain offered Greece "territorial concessions in Asia Minor" if it would participate in the upcoming Gallipoli Campaign. Venizelos supported this idea, while the king and his military advisers opposed it. Dismayed by the king's opposition, the prime minister resigned on 21 February 1915. A few months later, Venizelos' Liberal Party won the May elections and formed a new government. When Bulgaria mobilized against Serbia in September 1915, Venizelos ordered a Greek counter-mobilization and asked the Anglo-French army to defend Thessaloniki and aid Serbia. The Allies, led by General Maurice Sarrail, began landing on 22 September 1915 and entrenched around the city. The Greek parliament gave Venizelos a vote of confidence to help Serbia, yet the king unconstitutionally dismissed the prime minister along with the parliament. This unlawful order escalated the animosity between the king and Venizelos as well as their loyal followers. The Liberals boycotted the December elections.

On 9 May 1916, Erich von Falkenhayn, Chief of the German General Staff, informed Athens of the imminent advance of German-Bulgarian forces. In reply, Athens minimized the importance of General Sarrail's movements and requested Falkenhayn to change his strategy. On 23 May, Falkenhayn guaranteed that Greece's territorial integrity and its citizens' rights would be respected. On 26 May, despite an official protest by the Greek government, 25,000 Bulgarian soldiers led by German cavalry invaded Greece. The Greek forces at Fort Roupel unconditionally surrendered. The German Supreme Command was concerned about Allied General Sarrail's movements, and Falkenhayn was ordered to occupy strategic positions inside Greek territory, specifically Fort Roupel. Despite the assurances of Falkenhayn, Bulgarian soldiers immediately began to forcibly round up the Greek population into large cities, namely Serres, Drama and Kavala. German attempts to restrain Bulgarian territorial ambitions were partially successful, yet on 4 September, Kavala was occupied by the Bulgarian Army.