You need to give incentive to attack people.
I wiped the #1 account's army and what did I get in return?
Nothing!

I suggest a system of protection money like the mob. If you dominate someone's account you begin to siphon off some of their production until the debt is paid. You are now farming the account you beat in combat, and it's up to you to defend that account from other attackers.

The account in question could refuse to pay you the protection money after the fact, but many will not choose this option because paying say a 20% protection fee would be more worth it than to continue fighting.