In 305, Antigonus Monophthalmus, one of the Diadochi (successors of Alexander the Great), sent out his son Demetrius to capture the city and island of Rhodes. It controlled the entrance to the Aegean Sea, and its capture was necessary if Antigonus wanted to liberate Greece and conquer Macedonia.
Diodorus of Sicily tells about the siege in his World History 20.81 and 20.100. The translation was made by M.M. Austin.
The Siege of Rhodes
[20.81.1] After this year Euxenippus became archon at Athensnote[The year 305/304.] ... During his year of office war broke out between the Rhodians and Antigonus for more or less the following reasons.
[20.81.2] The city of Rhodes had a powerful navy and enjoyed the finest government in Greece, and so was an object of competition between the dynasts and kings, as each sought to win it over to his friendship. Seeing ahead where its advantage lay, it concluded friendship with each of the protagonists separately and took no part in the wars the dynasts fought against each other.note[As a mercantile state, Rhodes benefited most from peace.]
[20.81.3] And so it happened that it was honored by each of them with royal presents, and prospered greatly by remaining at peace for a long time. It had reached such a peak of power that it took up on its own, on behalf of the Greeks, the war against the pirates and cleared the sea of that scourge. Alexander, the most powerful man in human memory, honored it above all cities, deposited there his will concerning the whole kingdomnote[This is fiction.] and in general admired it and enhanced its preeminence.
[20.81.4] The Rhodians, then, by establishing friendship with all the dynasts, kept themselves immune from any justifiable complaint, but their sympathies inclined most towards Ptolemy. For it so happened that they derived the majority of their revenues from the merchants sailing to Egypt and that in general their city was sustained by that kingdom.
[20.100.1] The Rhodians, then, brought the war to a close after a siege of one year. They honored with appropriate presents those who had shown bravery in the face of danger and conferred freedom and citizenship on the slaves who had displayed courage.
[20.100.2] They also set up statues of king Cassander and king Lysimachus, who although they held second place in the general estimation had yet made a great contribution to the salvation of the city.note[Cassander and Lysimachus had sent blockade-runners. The most famous statue erected after the siege was not dedicated to these kings, but to the Sun: the famous Colossus of Rhodes.]
[20.100.3] But as for Ptolemy, they wished to repay his favor with an even greater one, and sent sacred ambassadors to Africa to ask the oracle of Ammon whether he advised the Rhodians to honor Ptolemy as a god.
[20.100.4] When the oracle had given its assent they consecrated a square enclosure in the city, which they called the Ptolemaeum, and constructed on each of its sides a galery 200 meters long. They also rebuilt the theater, the parts of the wall that had collapsed and the other buildings that had been destroyed, all far more beautifully than before.