Are there historical examples of countries or kingdoms that did a complete 180 on their old allies - not counting vassals or similars changing one master for another?
We don't have an Ask Historians yet, so I think this is the best place to ask at the moment
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.