There are a few settings you can play with.
For one thing, cards. If you make the cards more powerful, it increases the incentive to keep attacking. If a player doesn't attack, they don't get cards and will quickly fall behind another player that's getting cards.
Another thing to try is the [army cap](http://wiki.warlight.net/index.php/Army_Cap
). This puts an upper limit on the number of armies you can stockpile - once you reach the limit, you stop getting new armies and are forced to attack someone if you want to keep growing.