ETYM Latin expeditio: cf.French expédition.
1. A journey organized for a particular purpose.
2. A military campaign designed to achieve a specific objective in a foreign country; SYN. military expedition, hostile expedition.
3. An organized group of people undertaking a journey for a particular purpose.

Joint Allied (British, French, and US) operation 1918–19 to stabilize the Eastern Front following the Russian Revolution and safeguard the large concentration of military stores which had been sent to Archangel and other Western military interests in the area. The Allied force succeeded in capturing Murmansk July 1918 and then bombarded Archangel by air and sea, taking the city Aug 1918. The Bolsheviks were driven from the area and a new local government established. However by early 1919, Archangel was little more than an enclave in an otherwise almost entirely Bolshevik country and it was evacuated Aug–Sept 1919.
The expedition had three main objectives: to safeguard the large concentration of military stores which had been sent to the Russians and which lay in Archangel; to safeguard the flank of the Murmansk Expedition; and to try to stabilize the Eastern Front and make contact with the Czech Legion and the White Russian forces of Admiral Alexander Kolchak.
A mixed force of British, French, and US troops occupied Murmansk July 1918. Using this as a base, the Allied naval squadron, with air support, attacked and captured Modiuga Island, some 48 km/30 mi north of Archangel, which allowed them to bypass Bolshevik defenses and enter Archangel. A land force then cleared the valleys of the rivers Dvina and Vaga and defeated the Bolshevik forces which had been occupying Archangel. A local government was formed and formally recognized and several thousand Russians enlisted with the Allies.
After the Armistice, the Bolsheviks were able to concentrate troops in the area and by early 1919 there seemed little point in holding on to this enclave in a country which had almost entirely become Bolshevik. Archangel was successfully defended until Aug, when withdrawal began, and the British naval base was finally closed and the last troops evacuated 27 Sept, having handed their equipment over to White Russian forces.

In World War I, joint Anglo-French expedition to help Greece against Bulgaria Oct 1915, which remained in the area, reinforced by Serbs, until the end of the war.
In Sept 1915, fearful of Bulgaria's intentions, Greece requested a reinforcement of 150,000 troops from the British and French governments. The Allies agreed and the first troops landed at Salonika 3 Oct 1915, including British troops from Gallipoli. By the end of Oct there were about 40,000 troops in Salonika, mostly French. On Oct 11 Bulgaria invaded Serbia and the Allies advanced to make contact with the S Serbian army and halt the invasion. They failed due to the greater numbers of the Bulgarian forces and the Allies retreated back into Greece 12 Dec.
Despite having been of no assistance to the Serbians, it was decided to maintain a force in Salonika and the expedition was reinforced: by summer 1916 it consisted of 300,000 Allied troops and 100,000 Serbians plus elements of Russian and Italian troops. In July this force advanced up the Struma valley, the French linking up with the Serbians near Vardar. The Bulgarians attacked the center of the Allied line but in spite of some local successes they were eventually driven back, with the Serbians and French pressing on their right and the British turning their left flank.
Monastir fell to the Allies Nov 1916, after which the lines stabilized and trench warfare ensued until the spring of 1918 when General Franchet d'Esperey took command of the Allied forces in the area. By this time the Greek Army had been reorganized and had 150,000 troops available, and after careful planning a fresh offensive was begun Sept 1918 which, within two weeks, obtained Bulgaria's unconditional surrender.
After this the Allied force cleared all German and Austrian troops from the Balkans, and on 9 Nov a strong Serbian force crossed the Danube and was headed for Bucharest when the war ended.

(Used of military forces) Designed for military operations abroad