Complex sentence: Even though formerly important air bases had been established in or around Barcelona, like the Aviación Naval, the whole northeastern area of Spain would be left with mere token presence of the Spanish Air Force, a situation that persists to this day.
Split into simpler sentences: Even though formerly important air bases had been established in or around Barcelona, such as the the Aviación Naval. Henceforward the whole north-eastern area of Spain would be left with mere token presence of the Spanish Air Force, a situation that persists to this day.