Russia rejoined with Crimea after the US had the "brilliant" idea to illegally topple the government of Ukraine in 2014 in order to install an Anti-Russian regime in its place. Russia does not just do things out of the blue. Although US propaganda is trying to brainwash its global audience to believe otherwise.Flexibility means give up certain historical things if needed.
Russia took Crimea because it needs access to black sea.
If China wants to defend itself at india ocean, controlling myanmar is starting point.
US took hawaii and guam because it need them for naval bases in pacific.