For starters each of their ground troops have the phyical stats to casually move in the bottom of the marianes trench, where the pressure alone would crush tanks like tin.
They have complete control of the majority of the planet, and all coastlines would fall fast.
They have giant magical sea monsters.
Important nations could be sunk easily by the trident of neptune.
I don't know if the whole military can do a thing to aquaman on his own.
Atlantis has the complete advantage.