I honestly wouldn't consider anything until the turn of the 20th century the harbinger of Modern Warfare. It isn't until the Second Boer War really that you start to see an analysis of small unit tactics and a break away from big conventional armies in formations.

Obviously groups like the Zulu and Maori have used irregular tactics for centuries, but on a global scale, it was really the Second Boer War that brought global attention to small infantry tactics and when people started looking into "ok, how do we operate with a squad/platoon in close quarters and adapt to unorthodox maneuvers" rather than "how do i organize this company in ranks and columns and follow a set battle drill?"

Then obviously we have WWI and Rommel's Infantry Attacks. That's when small unit tactics really started to become the basis of modern warfare.