I think it's the "defensive.ai" default AI, or rather, if you're referring to majors, the country-specific defensive AIs.
For example, after Germany has occupied most territory in European Russia, Soviet Union gets a "holy crap we're gonna die AI" that makes it produce masses of militia and nothing else. Same for Italy et al.
EDIT:
for Italy: Allies control North Africa and land in mainland Italy.
for Germany: USSR controls Warzow (sp?), Allies control Paris, and the US has entered the war.
for Japan: US lands on main islands.
for France: at war with Germany.
AFAIK these are the only ones with defensive militia-mad AIs, not sure about other countries.