William H. Riker. 1964. Federalism: Origin, Operation, Significance. Boston: Little, Brown and Company. Chapter 2 (“The Origins and Purposes of Federalism”), particularly pp. 11-16.
The first condition of federalism is the willingness of politicians to bargain to expand territorial control. The second is the the acceptance of the bargain by rulers of constituent units in response to some external military-diplomatic threat or opportunity.
The circumstances that have resulted in federal rather than unitary constitutions can be characterized by a recognized need for military-diplomatic unity and an unwillingness or inability of national leaders to impose centralization by force.
Military and expansion conditions are necessary to the occurrence of federalism.