Could the Allied force sent to Gallipoli have saved Constantinople if transported to 1453?
Short answerIt could probably defeat the besieging army at first, but long-term success would depend on industrial supplies, disease control, command cohesion, and politics.
NotesThe Gallipoli force's artillery, rifles, machine guns, and naval support would overwhelm a fifteenth-century assault in the short term. Its real difficulty would be sustaining coal, petroleum, ammunition, spare parts, food, and medical support once the initial stocks ran down. The troops also came from several countries and could not be assumed to accept a new political purpose indefinitely. A successful defense of Constantinople would therefore be followed by hard questions about authority, relations with Byzantium, and the force's ability to remain supplied.