M'mah nafshach: If they were able to defend themselves, wouldn't they have done so without Achashveirosh's permission? Surely they would not have gone like sheep to slaughter! And if they did not have any means of defense, what good does a letter do?
Rav Amiel answers in his Derashos El Ami that the people from day one had the means of defense. What they lacked was fortitude and resolve. Kiymu v'kiblu, Esther's revelation of her identity, and finally, the letters of Achashveirosh provided the psychological boost, the necessary chizuk, for the people to take action.
There is something a little sad here. Why do we need a letter from Achashveirosh, from Lord Balfour, from the UN, from Trump, to empower us to fight for our rights? Ha'levai it should be kiymu v'kiblu alone that gives us that strength.