Complex sentence: The month marked the nadir of the RFC's fortunes, however, within a couple of months the new technologically advanced generation of fighters (the SE5, Sopwith Camel, and Spad XIII) were gaining ascendency over the Jastas.
Split into simpler sentences: The month marked the nadir of the RFC's fortunes. However, within a couple of months the new technologically advanced generation of fighters (the SE5, Sopwith Camel, and Spad XIII) were gaining ascendency over the Jastas.