The United States of America currently possesses approximately 4760 nuclear weapons, zero of which are capable of being used to fight alien space ships.
There are many reasons nations cannot use their nuclear weapons, a major reason being that they would face a reactive counter-attack from the allies of the nation they used the nuclear weapons against. However aliens attacking us do not face such repercussions because we have no way to return fire.
We must equip our nuclear missiles with interplanetary capabilities in order to better defend ourselves from possible alien threats.



