Complex sentence: Under Sancho III "the Great" (1000--1035), Pamplona controlled the entire southern Basque Country; indeed, its power extended from Burgos and Santander to northern Aragon.
Split into simpler sentences: Under Sancho III "the Great" (1000--1035), Pamplona controlled the entire southern Basque Country. Actually, its power extended from Burgos and Santander to northern Aragon.