Complex sentence: They were mainly untrained naval recruits, and he was criticised when over 2500 were interned or became casualties, but they had prolonged the defence of Antwerp for several days, perhaps a week, and they almost certainly enabled Dunkirk and Calais to be secured.
Split into simpler sentences: They were mainly untrained naval recruits, and he was criticised when over 2,500 were interned or became casualties. But they had prolonged the defence of Antwerp for several days, perhaps a week, and almost certainly enabled Dunkirk and Calais to be secured.