The Mediterranean Expeditionary Force (MEF) was part of the British Army during World War I, that commanded all Allied forces at Gallipoli and later Salonika. Its headquarters was formed in March 1915. The MEF was originally commanded by General Sir Ian Hamilton until he was dismissed due to the failure of the August Offensive. Command briefly passed to General William Birdwood, commander of the Australian and New Zealand Army Corps, but for the remainder of the Gallipoli campaign it was General Sir Charles Monro who led the MEF.

Supposedly when the British Secretary of State for War, Lord Kitchener, was preparing the Mediterranean expedition he intended to name the headquarters the 'Constantinople Expeditionary Force' but Hamilton suggested this might be a bit of a giveaway, and also noted in his diary, "I begged him to alter this to avert Fate's evil eye."

IMPORTANT NOTE: There are more units being added to this over the coming months to include the Army Service Corps, Royal Army Medical Corps, Royal Artillery, Royal Engineers etc.