Why do nuclear-armed countries still seek to guarantee their security?

Normally, nuclear weapons should guarantee the inviolability of any country that possesses them. a country that dares to attack and invade the territory of a nuclear-armed country runs the risk of destruction, even mutual destruction if the attacking country itself possesses nuclear weapons.

For these reasons, I don't believe that nuclear-armed countries can really see their security threatened.
Why do nuclear-armed countries still seek to guarantee their security?
Why do nuclear-armed countries still seek to guarantee their security?
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