Complex sentence: Maj. Gen. Gale had initially intended for the 6th Airlanding Brigade, to which the 6th Airborne Armoured Reconnaissance Regiment was attached, to be landed first; however, aerial photography revealed that anti-glider poles had been erected in the landing zone selected for the brigade.
Split into simpler sentences: Maj. Gen. Gale had initially intended for the 6th Airlanding Brigade, to which the 6th Airborne Armoured Reconnaissance Regiment was attached, to be one of the brigades to be landed first. However, aerial photography revealed that anti-glider poles had been erected in the landing zone selected for the brigade.