Complex sentence: He did not come to play a major role as a naval officer; in June, he was ordered to seek to relieve Bornholm, but even before the issuing of the order the island had fallen into enemy hands.
Split into simpler sentences: He did not come to play a major role as a naval officer. In June, he was ordered to seek to relieve Bornholm, but he never got the opportunity to because the island had fallen into enemy hands.