Plan W

Plan W, during World War II, was a plan of joint military operations between the governments of Ireland and the United Kingdom devised between 1940 and 1942, to be executed in the event of an invasion of Ireland by Nazi Germany.

Although Ireland was officially neutral, after the German Blitzkriegs of 1939–40 that resulted in the defeat of Poland, the Low Countries and France, the British recognised that Germany planned an invasion of Britain (Operation Sea Lion) and were also concerned about the possibility of a German invasion of Ireland. German planning for Operation Green began in May 1940 and the British began intercepting communications about it in June. The British were interested in securing Ireland, as its capture by German forces would expose their western flank and provide a base of operations for the Luftwaffe in the Battle of the Atlantic and in any operations launched to invade Great Britain as part of Operation Sea Lion.

British-Irish co-operation was a controversial proposal for both sides, as most members of the Irish political establishment had been combatants in the Irish War of Independence between 1919 and 1921. However, because of the threat of German occupation and seizure of Ireland and especially the valuable Irish ports, Plan W was developed. Northern Ireland was to serve as the base of a new British Expeditionary Force that would move across the Irish border to repel the invaders from any beach-head established by German paratroopers. In addition, coordinated actions of the Royal Air Force and Royal Navy were planned to repel German air and sea invasion. According to a restricted file prepared by the British Army's "Q" Movements Transport Control in Belfast, the British would not have crossed the border "until invited to do so by the Irish Government," and it is not clear who would have had the operational authority over the British troops invited into the State by Éamon de Valera. The document added that most people in Ireland probably would have helped the British Army, but "there would have been a small disaffected element capable of considerable guerrilla activities against the British."

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