Here's some quick and easy techniques to use in times of war.
Present a confusing front:
Enemies tend to fortify the member being scouted or attacked at that moment. So target multiple league cities with only one soldier and deploy your full force to your main city. This increases the chance to attack a city that isn't fortified to the full support of the league. The same method can and should be used with scouting to make the main target indeterminable.
As the water does on the shore's break:
As your army gets bigger it may seem to be better worth to send your full force down upon the enemy. But rather it is more effective if you send your armies by waves. This will more effectively chip away at the enemies morale. What is worse a city full of dead after one attack or a mailbox full messages of constant war?
A snake with no head coils into itself:
Diplomacy, my favorite method of warfare and in my belief the most powerful. If we convert or take away the officers of the league through diplomacy, most organization within the league is lost. Precious time and effort is wasted in electing new leaders and reorganizing the league under their order.
A head with no body can only talk and bite his tongue:
With the combination of the three previous techniques and the addition of propagandist messages, that questions the authority of their leader, we can hopefully reduce the numbers of loyalist in the league and their overall membership. Without a vast force to support him the leagues leader is vulnerable to our attack.