A few other games I have played like this has a rally building.
It is similar to a muster point, in the sense that the higher the rally building level the greater the amount of troops that can rally.

It's purpose? You find a target. You select it and press rally. A timer starts eg 2 hours. Your allies can send troops within the 2 hours to the rally. Once the timer finishes. The march to the destination begins with all troops that rallied. Defender then gets a report for all involved.

Level 1 - 25000
Level 2 - 50000
Level 20 - 500000

Basically defenders can get reinforcements. Attackers should be able to as well. Also encourages more team work from alliances.