It took Xerxes four years to plan and prepare for his massive invasion of Greece. From every corner of the Persian Empire, Xerxes collected vast quantities of food, money, and weapons, which he stockpiled for the impending conquest. Meanwhile, the Persian army honed its skills by putting down a rebellion in Egypt.
Xerxes's army was the biggest that had ever been assembled in the ancient world, numbering at least several hundred thousand troops. The army's strongest regiment was an elite corps of ten thousand hand-picked warriors known as the Immortals. The greater part of the army was, however, drafted from the enslaved masses of the Persian Empire.
This army began marching toward Greece in the spring of 480 B.C. It took a full week for the entire force to pass by any given point along the line of march. As the thirsty army passed through the countryside, springs, wells and small rivers along its route were frequently drunk dry.
Xerxes ordered a fleet of a thousand warships to sail along the coast of the Aegean Sea, following his army and carrying provisions for the long march to Greece.
Xerxes called a temporary halt when his army reached the stormy straits of the Dardanelles, at the mouth of the Black Sea. To transport his army across the mile-wide waterway, Xerxes sent hundreds of ships from his fleet into the channel, where they were tied together with thick ropes to create a floating bridge. A violent storm destroyed this bridge before the Persian army was able to cross it.
Enraged, Xerxes ordered that the waters of the Dardanelles be whipped and branded with hot irons. The soldiers sent to perform this symbolic act of punishment recited the following royal proclamation:
"O vile waterway! Xerxes lays on you this punishment because you have offended him, although he has done you no wrong! The great king Xerxes will cross you even without your permission, for you are a treacherous and foul river!"
Xerxes also put to death the engineers responsible for building the bridge. He then recruited a second team of bridge-builders, who cleverly decided to build two floating bridges, one for the use of the army's troops, and a second, downstream, which would carry the large herds of horses and other animals across the straits. The engineers also made sure to use thicker ropes to tie the ships together. After the ropes were pulled taut by giant windlasses anchored to either shore, mile-long embankments of timber, stone and packed earth were laid across the ships' decks to form roadways.
In the weeks that it took to assemble the floating bridges, Xerxes ordered his naval commanders to allow grain-ships bound for Greece to pass through the Dardanelles unharmed. "Are we not bound for the same destination?" he asked. "I do not see that those ships are doing us any harm by carrying our grain for us."
When the bridges were completed, Xerxes and his army crossed the straits into the kingdom of Thrace.
