Ultimate magic item in a war is...

[megamania]

First thought was a wand of fireball or staff of similiar power. Always effective against an army.
If money is not an object- Various summoning spells would be nice especially with augumented feats to boost the summoned help.
Elemental aid is good.
Clerics that can command/rebuke many undead would not be bad.
There are (were in 3.0) many nasty psionic powers designed for dealing with large numbers of undead.

If the army is TOO big. Relocate.

 

[humble minion]

A Mirror of Opposition. You might need to enlarge it, protect it, and get someone to Telekinesis it around the battlefield, but it'll do the job.

 

[Umbra]

The Holy Hand Grenade of Antioch.

With instructions, of course!