@IceVoidSoldier
@Chaeden
You can't use your trump cards too early. The ring likely isn't something that lasts forever - enough to fight one battle group maybe and not more.
If Earth had used the ring in an earlier fight, he would have wiped out maybe a dozen dragons, and then now he'd be unable to use it in this fight.
If Gehenna Cross has any reinforcements that can pull off and attack later, using the trump card now would potentially leave the reserve force able to defeat the weakened army.
Using a trump card is something you pull out when you're absolutely sure the enemy's engaged sufficient forces that you can defeat them all in one go.
(Sure, people might blame him, but there's definitely a reason for not pulling it out earlier.)