What lead the Rhodians to build the Colossus?
After the death of Alexander III of Macedon (Alexander the Great) many of his generals, friends and family fought great battles over who would be King next. These people were called the Diadochi or 'successors'. The Greek island of Rhodes is positioned at the entrance of Aegean sea. Their strong navy meant that they controlled vast amounts of ocean but they did not choose a side in the Wars of the Diadochi because they wanted to have trade with everyone. Being unaffected by the war meant the Rhodes grew very rich and it's wise government chose to work towards cleansing the sea of pirates. However, the Greek of Rhodes remained neutral but was sympathetic to one general named Ptolemy I Soter and traded mostly with him.
Ptolemy was a child-hood friend of Alexander III and his mother was called Arsinoe of Macedon. Some stories claim that Arsinoe had at one point been a concubine of Alexander III's father Philip II. Philip later apparently gave her to a man named Lagus and so it is unclear which man was Ptolemy's father. After Alexander III's death, Ptolemy came to rule Egypt.
Another man who wanted Alexander's throne was Antigonus I Monophthalmus (Antigonus one-eye) who had been a general of Alexander the Great and had been the protector of his supply lines. Back when Rhodes had been under the command of Persia, many Greek mercenaries served there under the command of Memnon of Rhodes. When Memnon lead his mercenaries against Alexander III's supply lines, Antigonus had been the one to stop him, despite Memnon's policy of burning everything useful to Alexander.
Once united under Alexander, now these two great generals were in competition and the prize was an empire. Antigonus One-Eye was allied to his son Demetrius I "the besieger of cities". Demetrius had fought against Ptolemy again and again since he was 22 and was now considered King along with his father. Now, just like his father had done before him, Demetrius was to attempt to take on Rhodes.
The attack on Rhodes was caused because Demetrius worried that Rhodes might sell ships to his enemy Ptolemy. Even in Demetrius's city of Macedonia, the attack was unpopular and considered to be in bad taste. This was partly because Demetrius allied his 200 ships and 150 auxiliary vessels with over 1000 pirate ships. The same pirates who Rhodes had been fighting against for early part of the Diadochi Wars.
Demetrius was enthusiastic to make sure that he had all of the most up-to-date equipment in his invasion, including his "Helepolis" which was a kind of siege-tower. The Helepolis had been invented by Polyidus of Thessaly, the Weapons Engineer of Alexander the Great's Father. It stood 41.1 meters high and 20.6 meters wide, making it the largest siege-tower ever. It rested on 8 wheels, each nearly 5 meters tall. It was plated in metal for fire-proof and had 16 catapults and 4 dart throwers and it required 3,400 men just to move it.
However, despite Demetrius's higher numbers and powerful war machines, The people of Rhodes proved victorious with Ptolemy's help in a single year, probably because of how unpopular the fight had become on Demetrius's side. In gratitude to Ptolemy, and under the advice of the Oracle of the sun god Amen in Egypt, the people of Rhodes decided to celebrate Ptolemy as a God (who also started calling himself Pharaoh of Egypt the same year) and they consecrated a square enclosure in the city, which they called the Ptolemaeum, and constructed on each of its sides a gallery 200 meters long. In celebration, the people of Rhodes melted down the Helepolis (which had been left behind) and sold much of it, using the money to pay for the creation of the Colossus of Rhodes, a giant statue of the Sun god Helios. Perhaps it amused them to take the weapon that would have destroyed them and show the world that the might of the very sun itself was on there side. Also to turn a HELepolis in the god HELios has a certain irony too.