We don’t have an Ask Historians yet, so I think this is the best place to ask at the moment
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If we take “old allies” not so much as old friends but more as “previously allied with:” Japan was on the side of the allies in WW1 but was axis in round 2. Nazi-Germany invaded the USSR in spite of their peace pact. Napoleon and tsar Alexander of Russia were on somewhat friendly terms before Alex clandestinely rejoined the coalition against the French.