Currently reading Alexander the Great. One of the things that interested me, was how especially earlier in his campaigns, was how he would change administration of a conquered nation. Simply he would remove the head of the nation, and get the rest of the administration to swear allegiance to him. Not really changing the mechanisms of how government is run.
I can't help contrast the invasion of Iraq where the US lead forces sacked everyone including the army, police; the place went into chaos.