This chapter begins the task of evaluating the evidence on how the laws of war work in practice by testing the hypotheses at the end of Chapter 3. The ratification of these treaties creates a common conjecture that both sides will follow them if war should break out. The threat of reciprocity enforces this shared expectation of restraint when self-control fails. The possibility of strategic advantage gained from breaking these rules along with noise produced by individual violations mean that these commitments will be broken by some parties in some wars. In response, the other side should respond in kind to these violations.

The key questions addressed in this chapter are:

1. Does the compliance of the warring parties move together?

2. How does joint ratification of treaties, which produces a common conjecture that the treaty rules will be followed, affect compliance and the correlation of the warring parties’ compliance?

3. Does unilateral ratification of a treaty standard change the pattern from joint ratification?

4. Does greater scope for individual violations across issues lead to less compliance?

5. Do warring parties respond more strongly to clear legal violations as opposed to violations whose legal status is open to question?

6. Are there firewalls between the different issue areas, so that violations by one side on one issue are not correlated with violations by the other side on different issues?

7. Do defections from treaty standards come early or late in wars?