Mordechai was a Religious Zionist.
He opposed the party because Achasveirosh was celebrating that the Beit Hamikdosh was not rebuilt after 70 years per his calculation. Achashveirosh was not celebrating a conquest or his birthday, but what he thought was the end of Zionism.
After nearly 70 years of exile, the Jews war making skills had probably fallen into disuse. Under Mordechai’s leadership they had nearly one year to train for battle, and were successful in the battle of 13 Adar.