The Nuke is the only weapon that we, in the present time, have what the UNSC of the future has. Yes I just said it! In 500 years we still have the best weapon we ever created.
Now, when us humans are stuck against a seemingly invincible army of super-uber-evil ballchinnians with a great big plot to destroy the world we normally nuke them, and then pull off some witty remark like "bet they didn't see that coming" or something of the sort.
No real nuclear weapons are seen in Halo—instead, we humans opt for some form of MAC gun which shoots out Apple Macs and parts of Bill Gates which will blind the Covenant with their contraptions and their advancedness. Why would they call it the MAC? The Human Overlords designed it so the Microsoft computers will infect the Covenent network crashing it. So Basically, the MAC guns pretty much screw up EVERYONE'S plans, including the Master Chief's, although it does clear the way for more of the bad guys to run in and let him go shooty-shooty. The only time a nuke was "used" was the NOVA bomb detonation. Supposedly it wrecked most of a Covenent planet. While it does increase ratings for the show "Halo", in reality it wasn't a nuke. It was a fart, a massive fart, ignited with the power to shatter worlds. Maybe thats why NOVA stands for, Not a nuke O I like holes like O V stands for something And its a freaken
nukefart bomb. Thank that stupid Methane breathing fart lord for lighting the fart on fire with his grunty gas.