Take a bit more ancient example: the Greek phalanx. It was considered to be an excellent formation for quite a long time, and yet the way to beat it turned out to be trivial: use highly mobile light slingers to harass the unwieldy phalanx until it falls apart. If you happen to know that, you could be a very helpful adviser to a Greek (or Persian :-D) general a century or two earlier.

Peltasts were commonplace in Greek warfare; they didn't displace the phalanx, or prove the phalanx's weakness, they supported the phalanx formations. This is the issue; war is considerably more complex than a "Formation X beats formation Y" equation. Roman legions continued using variants of phalanx formations centuries after the Greek and Persian war.

That said, there is technology that could be brought back to revolutionize warfare: Logistics. Modern statistical methodologies would be an incredible asset. But being the guy calculating how much food to bring and when to send deliveries isn't as exciting.