Greatest and most effective military tactics and strategies?

Mar 2016
Scorched earth has generally done pretty well at stopping numerically superior forces invading a country, as much as it causes great destruction and suffering for the common people.
Oct 2011
Byzantine implementation of Fabian strategy against the Caliphate. They would retreat into forts, and let the Caliphate forces plunder the countryside - which turned out useless anyway, since economy of Asia Minor was based on the animal husbandry, and unlike crops, animals have legs.

A lot of Byzantine strategy was actually based on the experiences of Emperors Maurice and Heraclius. Relevant: