There are termite species that do not have a soldier caste at all, and they are fairly well represented in tropical regions of the world, especially South America and Africa. These soldierless termites are so far only known from a couple of subfamilies in the large Termitidae family.
Apicotermes, Anoplotermes, Protohamitermes, and Invasitermes are some of the genera that I am aware of that do not have any soldiers. Instead, the workers take on that role by resorting to using their gut contents to either 1) seal up nest damage, or 2) trap invading ants.
At first glance, it may appear that their defensive method is inadequate, as casualties are often high on the termite’s side, especially in the event that a large breach has occurred and ants manage to invade en masse. However, most of these soldierless termite species feed on decaying wood or humus, and in their niches, largely avoid contact with ants while foraging. Apparently, the lack of soldiers does not hinder them from thriving in their forest environment.
Many soldierless termites have a form of defense called autothysis. You could call it a “kamikaze” form of defense, in which the defender is able to rupture its internal organs and spill out its gut contents that are either sticky or poisonous, onto an ant. In the narrow confines of their tunnels, these termites can therefore seal off the tunnels with their bodies (and those of the dead ants that they entrap). An example of a species that adopts such a defensive method is Globitermes sulphureus (they do have a soldier caste).
In the Oriental region, soldierless termites are rarer, but there is one species that I did come across – Protohamitermes globiceps. Not much is known about them, but they appear to nest inside, and feed on, decaying wood, and are considered to be a monotypic genus, meaning there is only one species for the entire genus. Soldiers have never been found for this species.