Until this year, the earliest known warrior termites were from the Miocene. See Engel et al. (2016).

See: http://www.nature.com/nature/journal/v530/n7590/full/530256a.html?foxtrotcallback=true

"Insects that are “eusocial” live in colonies with closely related nestmates and display social behavior, including a division of labor. The best-known examples are honey bees, termites, and ants. All eusocial insects display the following three traits: 1) They cooperate while caring for their young, 2) there is a division of labor among different castes in the colony or nest, and 3) at any given time, at least two generations are present and active. Eusociality is thought to have appeared first in termites in the Late Jurassic, about 150-160 million years ago, but the earliest termite fossils that could definitively be tied to a caste system were from the Miocene, a mere 20 to 17 million years ago. A similar story held true for ants, whose evolutionary history with eusociality was also thought to be long, but only weakly supported by the fossil record. However, scientists recently found fossils that indicate that eusociality existed 80 million years earlier. Ants and termites that were recently discovered in 100-million-year-old amber provide direct evidence of advanced social behavior. The new work, led by scientists at the American Museum of Natural History and the University of Kansas, was published in two papers in the journal Current Biology."