De Valera and anti-treaty Sinn Feiners ended their boycott of the Irish legislature following the national election in 1927. They entered the D�il as members of the opposition in the newly founded Fianna F�il Party. Fianna F�il gained control of the D�il in the 1932 national election, partly as a result of the Cosgrave government�s inability to cope with economic problems brought on by the world economic downturn of the early 1930s. De Valera became president of the executive council (prime minister) of the Irish Free State and would remain a powerful force in government for decades. Under de Valera�s leadership, Ireland would become a more nationalist, isolationist, and inward-looking society.

Once in office, de Valera embarked on a systematic program to eliminate British influence in Irish affairs. His government revoked the oath of allegiance to the British crown and ignored the governor-general in Ireland. De Valera also backed legislation to suspend payments of land annuities owed to the United Kingdom, sparking a tariff war between the two countries that would last until 1938.

Although high tariffs levied by Ireland on British goods were largely retaliatory, they were also part of a broader set of measures supported by de Valera to shield domestic industry from international competition and give Ireland a self-sufficient economy. Other steps taken included the establishment of high income taxes on the wealthy and strict controls on foreign capital invested in Ireland. Economic nationalism was matched by a policy of cultural nationalism, which placed the Roman Catholic religion, the Irish (Gaelic) language, and Gaelic sport at the center of Irish identity.

In 1936, following the abdication of British king Edward VIII, de Valera�s government successfully abolished the office of governor-general and deleted all references to the British monarch from the constitution of the Irish Free State. The External Relations Act of 1936, passed at the same time, limited the Free State�s association with the Commonwealth of Nations to joint action on a limited number of foreign policy matters.

De Valera and Fianna F�il won reelection in the July 1937 national election, and in a simultaneous plebiscite, Irish voters approved a new constitution proposed by de Valera. This document renamed the country �ire (Gaelic for �Ireland�) and established a �democratic state, sovereign and independent.�

The new constitution provided for an elected president as head of state, a prime minister as head of government, and a two-house legislature. The constitution formally claimed authority over all of Ireland, although its application in Northern Ireland was not to take effect until after reunification. The constitution made no reference to the British monarch or to the Commonwealth of Nations. However, de Valera indicated that �ire�s relations with the United Kingdom would continue to be governed by the 1936 External Relations Act, which preserved a formal role for the British monarch in Ireland�s diplomatic dealings with other nations. The constitution of 1937 has been in force ever since, although subsequent amendments have modified the document to loosen antiabortion laws, legalize divorce, and remove articles claiming territorial jurisdiction over Northern Ireland.

In 1938 Irish writer and patriot Douglas Hyde became the first president of �ire, with de Valera continuing as prime minister. In the same year, a treaty between �ire and the United Kingdom ended the costly tariff war. The treaty provided for the withdrawal of British forces from naval bases in �ire in exchange for a lump-sum payment to settle debts owed to the United Kingdom. However, the slight improvement in relations between the two nations was marred by a violent terrorist campaign in the United Kingdom conducted by the IRA.

During World War II (1939-1945) de Valera followed a policy of strict neutrality for Ireland and refused to let British forces use Irish naval bases, despite German air raids on the city of Dublin in 1941. Although public opinion in �ire remained solidly anti-British, many of its citizens enlisted in the British armed services or worked in war industries in the United Kingdom and Northern Ireland. �ire, which emerged from the war relatively unscathed, was one of the few countries of Europe to avoid food rationing during the war. In the immediate postwar period, however, economic devastation in the United Kingdom and Europe subjected the economy of �ire to severe strain, resulting in a period of rapid inflation.