The alternatives are that neither European Axis power declares war, Italy alone holds back, or Italy declares while Germany does not. Each branch changes a decision while retaining the circumstances and people responsible for it. If the available evidence points toward one decision and a commander chooses another, the discrepancy can prompt a search for missing information. Personal motivations become an explanation to investigate after that possibility has been considered. The same method can compare a command decision with the information and doctrine available at the time. Applying it to December 1941 also requires attention to divisions within American politics.
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Alternate history as command analysis
Counterfactual comparisons can test whether missing evidence or personal motivations explain a decision that diverges from available information and doctrine.