The breakup of a flowing liquid jet surrounded by a second flowing liquid is governed by capillary instabilities. These instabilities are caused by minor perturbations in the flow field that build up sinusoidal waves at the jet interface and eventually break it up. The phenomenon of such fatal wavelengths was described first by Lord Rayleigh and later extended by several authors to quiescent and flowing liquid-liquid systems. The jet formation in a co-flowing environment was only recently investigated. Originating from the forced breakup of liquid jets in air, this paper reports on the breakup mechanism of liquid jets surrounded by a fluid. By using different flow cells and consequently different flow profiles along the jet trajectory, the breakup mechanism given by the material and process parameters could be excited or suppressed. Two questions concerning droplet generation and the droplet size distribution are addressed: the suppression of the fatal wavelength in transient flow fields and the use of external flow fields to support the fatal wavelength.