Complex sentence: Only two of the beaches (Juno and Gold) were linked on the first day, and all five bridgeheads were not connected until 12 June; however, the operation gained a foothold which the Allies gradually expanded over the coming months.
Split into simpler sentences: Only two of the beaches (Juno and Gold) were linked on the first day, and all five bridgeheads were not connected until 12 June. However, the operation gained a foothold that the Allies gradually expanded over the coming months.