This article explains variations in state war-propensity. I introduce a new typology of state war-proneness based on four major types of states: revisionist, failed, frontier, and status quo. The major novel contribution of this essay is the argument that the combined effect of variations in the extent of success in state-building (strong or weak states) and nation-building (nationally congruent or incongruent) shapes the level and the type of state violence by producing different categories of states with regard to their war-propensity. Strong states but nationally incongruent generate revisionist states, which initiate aggressive wars. The combination of state strength and national congruence leads to a status quo state. Weakness and incongruence bring about civil wars and foreign intervention in ‘failed’ states. Weakness but congruence produce the ‘frontier state’ with boundary and territorial wars, but also with a reasonable likelihood of evolution of status quo orientation over time. I focus here on key examples of these types of states, especially from two regions: Iraq and Lebanon in a highly war-prone region – the post-World War II Middle East; and Argentina and Brazil in a more peaceful one, at least in the 20th century – South America, although these states experienced quite a number of wars in the 19th century.