back-to-back
When you're totally surrounded by an enemy, and there's no escape, a trusted friend stands with his/her back touching yours, so you both can fight in efforts to protect each other's back. This can be done physically as well as verbally. The unity and dedication of the two coming together in this formation to act as one is a tactical defense whatever the battle may be.
At the Hot Gates (in the Hellespont), there were only two Spartans left against the Immortals who had encircled them. The two stood back-to-back defiantly in complete confidence of each other's capabilities and dared the enemy to attack, thus saving Western Civilization.