I dont see why people a still discussing this. The UNSC alone has the firepower to destroy the whole of the world. Longswords can reach mac 10 and preform accurate bombing runs at that speed at a much higher altitude. That's instantly faster than any current jet.

The UNSC does have artillery: The Rhino. 

UNSC guns are designed to fire more bullets than modern day guns, at faster speeds. 

MAC blast would take out an underground complex.