Complex sentence: When it was finally released late in the day, success was immeasurably more difficult, and even the 21st Panzer Division, which was able to counterattack earlier, was stymied by strong opposition that had been allowed to build at the beaches.
Split into simpler sentences: When it was finally released late in the day, success was immeasurably more difficult. Although the 21st Panzer Division, had counterattacked earlier but was stymied by strong opposition that had been allowed to build at the beaches.