How could zombies overwhelm a National Guard unit?

The Guard unit is in enclosed, claustrophobic urban terrain. A Humvee has broken down, and a couple of Guardsmen are desperately trying to fix it when the entire unit is set upon by a massive horde of zombies. Hundreds of them come from alleys, doorways, and numerous other points of entry. The Guardsmen don't see them until they are only a few feet away, and by then it is too late. The convoy is unable to move due to the wrecked vehicle, the few dismounted soldiers are being dragged away and torn apart, and the rest of the unit is desperately opening fire on every zombie they see. They make the mistake of aiming for center mass, when zombies can only be killed with headshots. An air strike results in horrendous friendly fire casualties. Burning soldiers stumble out of vehicles, only to be devoured in seconds while their remaining friends look on hopelessly. Soon, the entire unit has been overrun and killed.

*sigh*

Set up a beer tent with a sign "Free Beer for the Guard"...then surround them

The Guard loves beer...and free beer at that

But make the sign say "Freh Bere fo Gard"...because zombies don't spell so good

Superior numbers

File EO, SAPR, and IG complaints against them.

A pitch black night with no moon.

When they're sleeping?