Complex sentence: This impasse led to a blockade on Thessalonica, which occasionally flared up with direct attacks on the city, and a series of Ottoman raids against other Venetian territories, while the Venetians primarily aimed to block the passage of the Dardanelles at Gallipoli, with little success.
Split into simpler sentences: This impasse led to a blockade on Thessalonica, which occasionally flared up with direct attacks on the city. The Venetians repeatedly tried to apply pressure by blocking the passage of the Dardanelles at Gallipoli, with little success.