It turns out that in the face of overwhelming odds, even the protagonist's aura can't hold.
Though Shupard led the soldiers fiercely in battle, the Reaper attack came too swiftly and prepared too thoroughly. Now, Shupard and the others can only hide amidst the city's ruins, using the terrain to fight a defensive battle against the enemy. There's nowhere else they can go.
The sky is filled with those red spherical drones of the Reapers, and the Star Alliance's support can't reach them. Even with all the bravery and fearlessness of Shupard and her soldiers, they can't change the fact that they are running out of ammunition.
"Retreat, retreat!"