Decrees and Sufficient Time for the Jews to Save Themselves

Regarding Purim, it struck me as somewhat strange that the decree to kill the Jews would be issued almost an entire year before it was to be carried out...wouldn't this have given the Jews time to escape or ready themselves, regardless of a counter-order?

Haman made two decrees. The first was to destroy the entire Jewish People. The second was that everyone should be ready and prepared for something special on the 13th of Adar. The decree that the Jews should be killed was secret. The Vilna Gaon explains that the reason that Haman kept the first decree secret was in order to prevent the Jews from escaping in any way.

Sources:
• Esther 3:13,14
• Vilna Gaon, commentary to Megillat Esther