The Karlsruhe Tritium Neutrino Experiment (KATRIN) aims for a model-independent measurement of the neutrino mass scale with a sensitivity of 0.2 eV/c2 (90% confidence limit). This is made possible by using an ultrastable, high-luminosity windowless gaseous tritium source providing 1011 beta decays per second and a high-resolution integrating spectrometer with a resolution of <1 eV. Over the past years, the system was installed at the Tritium Laboratory Karlsruhe and commissioned in various stages while demonstrating the outstanding performance of the magnetic guiding, electron transmission, and stability of individual subsystems. In 2018, the KATRIN beamline was operated with traces of tritium for the very first time. In this campaign, first beta decay spectra could be recorded. This was essential to validate the physics model and the fitting methods of the KATRIN analysis. Furthermore, in the campaign it was demonstrated that the global KATRIN stability of 0.1% in this configuration was successfully reached. Based on these results—as well as those from a subsequent systematic calibration campaign—KATRIN is now performing neutrino mass measurement runs at nominal tritium purity.