The superior army
One of the main ironic elements presented in the play has to do with the fact that Xerxes' army was defeated even though it was better prepared, it had more soldiers and was better armed. Xerxes and the elders never thought their country will be defeated and were sure of their victory. The fact that the army and the King were so badly defeated is thus presented as an ironic idea.
The dead ruler would be a better one
Another ironic element has to do with the way in which Xerxes is seen by his own elders and subjects. Xerxes is always compared with his late father and he always comes up short. Ironically, many characters in the play express their wish to have Darius as a King, even if that meant having a corpse as a ruler.
Not my war
Xerxes is harshly criticized by outsiders based on the way in which he chose to handle the war. Xerxes is seen as incompetent because he did not manage to win and is constantly compared with his dead father. Ironically, Xerxes, as he points out at the end of the play, was not the one who started the war. In fact, the war against Sparta was planned by his late father and he was only following his orders.
The Persian army
The Persian army is hailed at the beginning of the play as being one of the most loyal and powerful armies all over the world. Because of the strength and the discipline of the armed forces, everyone was sure the Persians were going to win. Ironically, one of the things which Xerxes had to deal with was disobedience from his own soldiers. This proves that the Persian army was not as superior as everyone believed it to be.
The King is not mentioned
When the messenger came back from the battlefield, he rushed to meet the Queen Mother and to giver her the latest news. The messenger talks about a total defeat and then goes to read the names of those killed in battle or to talk about the fate of those who survived. Ironically, the one person who does not mention is the King himself, probably the most important person in the army.
