Revolutionary Éamon (pronounced 'Ay-mon') De Valera was Ireland's head of state during the Irish War of Independence (1919-1921).
In exchange for an end to the war and a guarantee of independece, De Valera's negotiator and fellow revolutionary - Michael Collins - agreed to give Britain uncontested control of Northern Ireland (a region constituting 1/6 of the Irish land mass and 1/3 of the population). Britain desired ownership of the North due to its large self-identified British population. Outraged by partition, De Valera and his allies initiated a Civil War against Collins' faction (1922-1923), which De Valera eventually won.
In 1939, World War 2 broke out. De Valera - still head of state - adopted military neutrality as his central foreign policy, wishing to keep Ireland out of the conflict. Fearing defeat during the Battle of Britain vs Nazi Germany, the UK Prime Minister, Winston Churchill, reached out to De Valera with the following offer: Ireland abandons it's neutrality and supports Britain during the war. In exchange, Britain commits to immediately return Northern Ireland, re-unifying the island.