Complex sentence: He first approached the Republic of Venice, by offering a military alliance and a sort of vassalhood to the Republic of Venice, but the Republic was then enjoying good relations with the Ottomans and thus refused Skanderbeg's offer.
Split into simpler sentences: He first approached the Republic of Venice offering a military alliance and a sort of vassal hood to the Venetians. But The Republic was enjoying good relations with the Ottomans and thus refused Skanderbeg's offer.