It depends on the scale of the entities at war and the lethality of weapons used. Soldiers take an awful lot of time to be born, bred and trained, and for each soldier you need an awful lot of civilians to spin the wheel of the economy. You can easily imagine regular low-scale skirmishes around a contested border for example, whereas a full-scale offensive would be too costly for either side(s) to mount, especially if there are more than 2 warring entities. Alliances allow more bloodshed, with supporting allies with no threat to their land providing material support for entities directly at war with each other.