Militaristic City-States grant units twice as often when you are at war with a common foe.
A united front is a system in which revolutionaries jettison state boundaries and pursue their objectives with other like-minded individuals in countries everywhere. The system was developed by communists seeking to spread the Bolshevik Revolution to countries across Europe and around the world. The battle cry was, "Workers of the world unite!," and the proponents hoped that the under-classes of the world's nations would realize that they had more in common with each other than they did with their own countries' rulers. The workers would rise up, overthrow their capitalist oppressors, and usher in a world-wide worker's paradise. This didn't actually happen and the movement was ultimately unsuccessful. However, the united front concept is seeing something of a resurgence in the ongoing cultural struggle between the Muslim and non-Muslim worlds.