Tingguians in the interior, Ilocanos in the north, Pangasinans in the south, and Zambals in the southwestern most areas settled the region.Before the administration of Ferdinand Marcos, Pangasinan was not a part of the region.
The Spanish arrived in the 16th century and established Christian missions and governmental institutions to control the native population and convert them to Catholicism. Present-day Vigan in Ilocos Sur province became the diocesan seat of Nueva Segovia. Ilocanos in the northern parts were less easily swayed, however, and remained an area filled with deep resentments against Spain. These resentments surfaced at various points in the Ilocos provinces’ history as insurrections, most notably that of Andres Malong and Palaris of Pangasinan, Diego Silang and his wife Gabriela Silang in 1764, and the Basi Revolt in the 19th century. However, it was the Pangasinans in the south who were the last to stand against the Spaniards.
In 1901, the region came under American colonial rule, and in 1941, under Japanese occupation.
During 1945, the combined American and the Philippine Commonwealth troops including with the Ilocano and Pangasinan guerillas liberated the Ilocos Region from Japanese forces during the Second World War.
Several modern presidents of the Republic of the Philippines hailed from the Region: Elpidio Quirino, Ferdinand Marcos, and Fidel V. Ramos. The province of Pangasinan was transferred by Ferdinand Marcos from Region III into Region I in 1973 and afterwards imposed a migration policy for Ilokanos into Pangasinan, to the moderate detriment of the native Pangasinenses. He also included Abra, Mountain Province, and Benguet in the Ilocos region in a bid to expand Ilokano influence amongst the ethnic peoples of the Cordilleras.
About content: https://en.wikipedia.org/wiki/Ilocos_Region